I.—On Didymograptus, Tetragraptus, and Phyllograptus

Extract

As has been previously indicated (see p. 439), the conclusion might be drawn from Moberg's description and figures of this species, interpreted by the knowledge of the true structure of the Didymograptus polypary, that the structure of the proximal part could in no essential particulars be separated from that just described in D. minutus, Mut. Consequently the genus Isograptus, which was proposed by Moberg for the species in question, is not authorized—at least, as founded on any of the characters cited by Moberg. The differences between Isograptus and Didymograptus, to which latter genus the species is referred by Nicholson, ought to be, according to Moberg, that in Didymograptus “both stipes arise at somewhat different levels on the sicula,” and that “each branch is itself bilaterally symmetrical”; whilst, on the contrary, in Isograptus the stipes should “arise bilaterally symmetrical from the sicula,” and each branch is not itself bilaterally symmetrical. As we have seen above, in reality in Didymograptus the left stipe is not bilaterally symmetrical, and the symmetry in the right stipe is entirely the same as that which is described by Moberg in Isograptus, namely, obliquely near the base, because the stipes do not arise from the side of the sicula, but from the “connecting” canal.