It is nearly forty years since Walcott published the well-known memoir which first gave palæontologists definite information regarding the appendages of Trilobites. This work was based on a laborious investigation of Calymene senaria and some other species by means of thin sections. Since then much knowledge has been gained, and, in particular, Beecher's researches on Triarthrus have provided us with a new conception of the Trilobite limb which, in some respects, is not readily to be reconciled with Walcott's earlier results. Now, at length, in this finely illustrated monograph, the veteran student of the Trilobita brings together the results of his own work and that of other investigators and reviews the whole in the light of his unrivalled experience.

In the year 1913 the author formed the intention of carrying out a comprehensive investigation of the mineral composition of the Lower Greensand strata of England, in order to ascertain whether any definite conclusions could be drawn as to the sources of the material and the geographical conditions that existed while the deposits were being formed. A large number of specimens were collected along the out crop from the Wash to the borders of Buckinghamshire and examined by the usual laboratory methods. This work was interrupted by the outbreak of war, and on returning to Cambridge after an absence of nearly four years on Government service circumstances proved to be unfavourable to a continuance of the work on the scale originally contemplated. The district already dealt with forms in itself a fairly well-defined unit, and it was decided to publish an account of the results already attained, in the hope that they may be of service to future workers in this interesting field of petrological and stratigraphical research.

When I received the invitation to open this discussion my first feeling was of diffidence, for, the interior of the earth being necessarily inaccessible to direct observation, the solution of the problems connected with it has principally been left to mathematical research, and this must remain the final court of appeal. In these circumstances it seemed verging on presumptuousness to address an audience consisting so largely of mathematicians in inauguration of a discussion on the interior of the earth. Second thoughts showed that there was much to he said against this view, for, though mathematics is the court of appeal, it can only decide on the facts placed before it by the sciences of observation, and so the discussion seems profitably prefaced by a statement of the leading facts which have been collected, and those conclusions which are so directly derived from them as to have almost the validity of observation.

The Corwen gravels were described by D. Mackintosh, forty-three years ago, in a paper “On the mode of Occurrence and Distribution of Beds of Drifted Coal near Corwen, North Wales”. He attributed the gravels to an interglacial marine submergence and claimed that the coal found in them was derived from the outcrop of Coal-measures near Kuabon, twelve miles due east of Corwen.

Conditions of Deposition — Former Presence of Sulphates — Formation of “ Demagnesified ” Rocks—Development of Concretionary and Segregated Structures, etc.

The largest, best exposed, and most interesting volcanic vent along the northern shores of Fife is that which has been laid bare at the headland of Kinkell Ness, and a portion of whose enclosed material has been sculptured into the picturesque shore stack known as the Rock and Spindle. (“These two words,” as Sir A. Geikie points out, “are generally misunderstood. ‘Rock’ is the Scots word for a distaff; and ‘Spindle’, as here used, has reference to the stellate mass of basalt resembling a spinning wheel.”) Reason for this appropriate designation will be readily gathered from the accompanying photograph (Plate XIII).

Essentially the pores of Megacystis are diplopores, for they are arranged in pairs. This is evident on the inner surface of a plate (E 16169, E 7671). Here each pore-pair lies in a shallow depression, and each pore again lies in a slight depression at the bottom of this (fig. 14). Compare Sinocystis yunnanensis (antea, p. 538).

During the last year I have examined, through the kindness of Mr. E. O. Forster Brown, Min. Eng., borings that have been put down by Messrs. Bell Bros., at Cotefield Close and Sheraton, near the southern limits of the Northumberland and Durham Coalfield. They are situated about 5 miles west of Hartlepool, and about 8½ miles east of the Permian escarpment at Ferryhill. The boring at Cotefield Close lies about 1½ miles east of Hurworth Burn station, and about the same distance south of that at Sheraton. The details of the former are from my own notes which were taken when the hole was finished, but those of the Sheraton cores are by Dr. C. T. Trechmann, who examined them as they were brought up from time to time, it thus being possible to obtain a very full record. I also desire to thank this geologist for some notes on the Sheraton boring which are incorporated in this paper. Both of the boreholes passed through the superficial deposits and Permian strata into the Coal-measures, but as that at Sheraton has just entered the lastnamed series and is not yet completed, the remarks on the Carboniferous rocks refer only to the strata pierced at Cotefield Close. The examination of these cores adds to our knowledge of the Permian, confirming some of the points that Dr. Trechmann and I have enunciated in previous papers dealing with the Magnesian Limestone.

The variability and occasional instability of the Ammonoid suture-line, to which attention has been drawn, the recurrence of similar types, and the frequent asymmetry of the opposing halves of a given suture-line, which is apparent not only in the Dactylioceras commune, figured by Swinnerton & Trueman (fig. 9 on p. 42), but also in the development of the suture-line in e.g. Pseudosageceras multilobatum, Noetling, in Indoceras baluchistanense, Noetling, or in Oxynoticeras oxynotum, Quenstedt, sp., to mention only a few well-illustrated examples, might be thought to impair the usefulness of the suture-line for the classification of Ammonoids. Yet, long before there was any subdivision of “Ammonites” at all, the greatest importance had been attached to the foldings of the suture-line, and Pictet stated in 1854 that “the lobes in their essential traits furnished very constant and very valuable characters”. Von Buch's group of “Arietes” was well characterized by the general plan of the suture-line, namely, the deep siphonal lobe and the short external saddle, only most authors would put more reliance on the ornamentation of the shell and put such a form as Asteroceras sagittarium, Blake, sp., into the genus “Aegoceras.” The writer would even go so far as to say that the type of suture-line given by Mr. Buckman for “Defossiceras” defossum, Simpson, sp., should not be found at the horizon stated, and that the form probably will turn out to be an Arietid (Agassiceras) of semicostatum age.

In addition to the variability of the suture-line in a given species, mentioned previously, asymmetry of the elements on opposite sides of the same suture-line is very frequent and probably universal in so far as the minor frillings are concerned, which is only to be expected in organic beings. This phenomenon has lately been illustrated again in Lioceras by Horn, and in Dactylioceras by Swinnerton & Trueman. The latter authors also have some interesting observations on asymmetry associated with lateral displacement of the siphuncle which is of sporadical occurrence in Ammonites.

The Dartmouth Slates of South Devon and Cornwall, which correspond, it would seem, to the Schistes d'Oignies of the Ardennes, are not seen in North Devon, but may be concealed by later rocks and be represented in South Wales and the Welsh Border by the Red Marls of the Lower Old Red Sandstone. It is possible that the Foreland Grits are a local facies of the upper portion of the Dartmouth Slates, just as the arenaceous Cosheston Group is a local development of the upper part of the Red Marls. Both the Foreland Grits and the Cosheston Group appear to have yielded the typical Old Red Sandstone plant Psilophyton.

In dissenting from Dr. C. T. Trechmann's procedure in rejecting the name of Clavigera for certain New Zealand Mesozoic Spirigerids and proposing the new name of Hectoria instead, I would like first to express the appreciation of New Zealand geologists for the painstaking work of Dr. Trechmann on the Trias of New Zealand.

The Diploporita, to which Order Sinocystis clearly belongs, have J been divided into Families according to the greater or less extension of the subvective system over the theca and the modifications thus induced in the arrangement of the thecal plates. All those in which the epithecal food-grooves do not extend beyond the adoral circlet of plates are referred to the Sphæronidæ. The line between the Sphæronidæ and the more advanced Glyptosphæridæ cannot be drawn rigidly, and such a genus as Proteocystis constitutes a natural transition. Sinocystis, however, comes well within this boundary.

The main feature of the geology of Drake's Island is the association of volcanic rocks with marine limestone of Mid-Devonian age. The Geological Survey memoir by Mr. W. A. E. Ussher attributes the discovery of the volcanic rocks to Mr. R. N. Worth. Mr. Ussher leaves their precise nature, “whether lavas or tuffs,” somewhat in doubt, but states that they are “most certainly contemporaneous”. As a matter of fact both lava and tuff occur, and are definitely distinguishable.

The field-work upon which this communication is based was carried out in 1917 and 1918, in preparation for the Geological Survey map of Sheet 86 (Huntly).

In Megacystis the periproct—which S. A. Miller always called the moutn—lies between adoral circlets II and III, being bounded by the two posterior facet-bearing plates and the posterior interradial, i.e. 3 of Adorals II, also by 2 or 3 of Adorals III, making 5 of 6 plates in all. The number 5 is the more usual; it occurs in M. aspera. faberi, gorbyi, indianensis, ornatissima, ornata, parcula, parca, perlonga, plena, scitulus, and spangleri, also in British Museum specimens E 7631, —33, —35, —37, —38, ?—39, —40, —42, —44, —45, —74, —76, —77, E16167, and E 16168 (see our figs. 24, 25, antea). The number 6, due to an additional Adoral III, is found in M. baculus, commoda, splendens, and perhaps gyrinus, also in E7630, —34, —36, ?—75, and E 16171 (see our figs. 22, 23, 28, 30, antea). This does not seem to be a difference of such constancy as to be relies on for the discrimination of species. It is the sole character distinguishing Miller's M. commoda from his M. gorbyi, not to mention M. scitulus and M. parva, which may be younger stages of the latter species. Similarly such specimens as E 7631, E 7635, E 7638, and E 7644, each with 5 plates, cannot otherwise be distinguished from E 7636 with 6 plates; while E 7630 and E 7634 with 6 plates are otherwise the same as E 7633, E 7640, E 7642, E 7677, and E 16168, each with 5 plates.

As an illustration of the difficulties encountered in basing the classification on some peculiarity of the Ammonoid suture-line the case of the two families Macroscaphitinæ and Crioceratinæ may again be referred to, the former of lytoceratid, the latter of hoplitid origin. Distinction between these two families was based on the bifid or trifid characters of the first lateral lobe. Hamulina nitida, v. Koenen, which shows very nearly equal-sized suture elements, has the trifid first lateral lobe of the type-species of Hamulina, namely S. dissimilis, d'Orbigny, but the plain shell of the lytoceratid Anahamulina. Hyatt put the latter into his family Macroscaphitidæ, but the former, and also the clearly lytoceratid Pictetia, into Ancyloceratidæ, i.e. even into a different sub-order. But Anahamulina subcylindrica, d'Orbigny, sp., i.e. the type-species itself, has a nearly trifid first lateral lobe, though it is connected through A. Lorioli, Uhlig, sp. (with a sub-bifid first lateral lobe), with typically lytoceratid forms. In ornament and coiling also Hamulina resembles certain lytoceratid forms (compare e.g. the various forms of Macroscaphitinæ figured by Uhlig).

‘Terebella’ cancellata was the name given to some supposed worm-tubes from the Cenomanian of the East and South-east of England, in the Geological Magazine for December, 1911 (p. 553, pi. xxiv, figs. 3-5). The chief feature of the species is an obscure cancellate ornament formed by transverse and longitudinal folds.

The Beer Stone is a gritty limestone, made up largely of shell fragments with some foraminifera, quartz grains, and 6oft chalky material, occurring in the Rhynchonella Cuvieri zone of the Middle Chalk near Beer Head in the south-east of Devon. It has long been worked for building purposes in underground galleries about one mile west of the village of Beer, and has been described by W. Hill and W. F. Hume in the Geological Survey memoir on the Lower and Middle Chalk. Dr. Hume records the following minerals as present in the insoluble residue of the stone: quartz, muscovite, glauconite, chalcedony, pyrites, tourmaline, rutile, andalusite, and possibly anatase. A. J. Jukes-Browne says the residue “contains a variety of minerals which have clearly been derived from land consisting of granite and Palaeozoic rocks such as occur in South Devon and Cornwall”.

The following notes were compiled, for the most part, some years ago, but their publication in the present form suggested itself to the writer on the perusal (during a short “leave” from active service) of a number of recent papers ou Ammonites, principally Professor Swinnerton and A. E. Trueman's study of the “Morphology and Development of the Ammonite Septum”. The main part of that inquiry is devoted to the development of the septum, illustrated by successive “septal sections”, and it is claimed that where sutural development cannot be worked out, “septal sections” will to some extent serve as a substitute. The writer lias no intention of discussing the usefulness of “septal sections”; but some of the suggestions put forward, and conclusions arrived at, by the authors, as well as certain opinions, whicli they adopt from other workers on Ammonites, invite critical examination. Since, in the present paper, other recent work on the morphology and physiology of the Ammonite septum and suture-line, not yet embodied in textbooks, is also included, and since the writer ventures to put forward opinions that differ in many essentials from the views of both textbooks and other authors, it is hoped that the paper may prove of general interest.