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Show 424 MR. J. W. HULKE ON THE SKELETAL [Nov. 20, arch (the expanded root of which, descending laterally on the noto-chordal sheath, represents a pleurocentrum), and of an inferior or ventral ossicle lying vertically beneath it, and so representing Gaudry's hypocentrum, there are also present distinct inferior ossicles in the notochordal sheath, intercalated one between each pair of composite vertebral bodies, and thus intruded between the hypocentra. Similarly superior intercalaries occur between the neural arches. To such inferior " intercalaria " the term intercentra is strictly pertinent. In the Ganoid Amia calva the cartilaginous tips of the transverse processes are structures having some correspondence to ribs. Now Dr. G. Baur mentions that in Amia calva the lateral (or transverse) process (Basalstumpf, Gdtte) at a certain point in the vertebral column, near the end of the body-cavity, passes from the centrum of a vertebra to the intercentrum next immediately following (30). In the only skeleton of Amia calva accessible to me (one prepared by Hyrtl preserved in the Museum of the Royal College of Surgeons), I find that behind the 6th vertebra following the body-cavity arch-less and arch-bearing centra alternate regularly ; and, except for a slight difference of size, these two kinds of centra are barely distinguishable. The lateral or transverse process, which in that part of the vertebral column which corresponds to the body-cavity is borne by the arched centra, alone present there, is not, in this skeleton, in the region behind the body-cavity transferred from the arch-bearing to the here intercalated archless centra (or intercentra) ; but the transverse process continues to occur orrly on the arch-bearing centra, until at the caudal end of the column, through reduction of bulk and through crowding, the distinctness of the component pieces of the column is lost. Ascending in the vertebrate scale, Hatteria, as shown by Dr. G. Baur, furnishes in its anterior vertebrae an example of the connection of a rib with a true intercentrum Here the capitulum of the furcate rib, mostly represented by ligament, is ligamentously connected with the intercentrum, whilst tbe tuberculum rests on the centrum. I find this arrangement present in the three anterior pairs of ribs in two skeletons of Hatteria now before me. The secondary connection of the ribs with the permanent vertebrae, and the arrangement in Hatteria demonstrating the connection of the capitulum costce and the intercentrum, would seem to favour the idea that the Crocodilian basilar piece is morphologically an intercentrum. The body of evidence, however, is I think, unfavourable to this conception ; and this, together with the fact that in the early embryo the basilar piece is continuous with the pars odontoidea and with the neurapophysis (including the hemicentroids, Albrecht), gives very great probability to the hypothesis that the basilar piece is really that which R. Owen termed it-the inferior part of the centrum of the atlas. This is also C. K. Hoffmann's view of it (31). The morphological equivalence of the Crocodilian basilar piece to the foremost of the subvertebral wedge-bones in Ichthyosaurus does not seem to m e proven, but rather the contrary. Probably in the Enaliosaur the " body " of the atlas is the equivalent of the Croco- |