IV.—On the Genera of Trilobites Asaphus and Ogygia and the Sub-Genus Ptychopyge

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By the able investigations of Mr. J. W. Salter, the various species of British Trilobites are now being systematically described, and referred to the genera to which they properly belong, the Asaphidœ with the rest; but there seems to be still some doubt as to the true differences between Asaphus and Ogygia. The feature apparently considered distinctive in the masterly work alluded to, is the form of the labrum; which, if furcate, is to be set down to Asaphus; if obtusely pointed, to be the distinguishing mark of Ogygia. How Ogygia peltata (Woodcut, Fig. 5) can be reconciled to this division, I do not see; but the main difficulty in the way of this classification is the sub-genus Ptychopyge of Angelin—distinguished by “having the facial suture within the margin in front.” This sub-genus Mr. Salter shows to be represented in Britain by Ogygia Corndensis, Murchison (Woodcut, Fig. 1): it is called Asaphus by Angelin, and has the wide axis, short and broad pleural furrows and simple caudal ribs of that genus. In addition to these features, the course of the facial suture finds a parallel' in the sub-genera of Asaphus, Isotelus, (Dekay) and Cryptonymus (Eichwald); Mr. Salter cites but one species of Ogygia that bears resemblance in this respect. Yet with all the stamp of an Asaphus, Ptychopyge Corndensis has the obtusely-pointed labrum common to Ogygia Buchii and O. Selwynii.