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Show 1888.] ANATOMY OF THE MESOSUCHIA. 421 That the pair of " lateral pieces" which, above, form the side-walls of the neural canal, below join the basilar piece, in front contribute to the supero-lateral part of the cup for the occipital condyle, which internally rest on the upper part of the antero-lateral aspect of the pars, encroaching slightly on the upper surface of this latter, are the morphological equivalents of the neurapophyses of other vertebrae is universally accepted. It is probable that they also comprise that part which in Mammalia, under the guise of the expanded root of the neurapophysis, contributes the dorso-antero-lateral portion of the body of the vertebra which P. Albrecht has named hemi-centxo'\d (17). This part of the atlas retains its individuality throughout the vertebral column in some early reptiles, of which Actinodon is an example. Gaudry, who has given excellent figures of the vertebrae of this Saurian in his admirable 'Enchainements,' very appropriately named this part pleuro-centrum (18) ; and this term has been adopted by E. D. Cope, who originally had designated the same part centrum in his accounts of Trimerorhachidians from homotaxic rocks in N. America (19). No part of the atlas has been the subject of more discussion than the azygos " basilar piece " which inferiorly completes the ring. Cuvier regarded this as the body of the atlas (20). R. Owen considered it to be "the inferior part of the centrum of the atlas" (21). He also regarded it as homologous with the ventral spur or carina present in the cervical and in the foremost thoracic vertebrae in extant Crocodiles, from which it differs, he remarked, in being autogenous. Further, this author identified it with the foremost of the " subvertebral wedge-bones" which in Ichthyosaurus supplements interiorly the atlantal cup for the occipital condyle (22). To the "subvertebral wedge-bones," to the ventral spur of the cervical vertebrae of extant Crocodilians, and to the Crocodilian atlantal basilar piece, R. Owen applies, alike to all, the term hypapophysis. But are all these morphologically equivalent structures, and is this term properly applicable to all ? Apparently R. Owen himself has not invariably used the term hypapophysis in the same sense, since he evidently has applied it to a part which in one instance is a downward extension of the centrum, and in another instance he has connected it with a part having an autogenous origin distinct from the centrum. Now in embryos of extant Crocodilians it is easily demonstrable that the ventral spur of the cervical vertebrae is a downward production of the centrum, with the tissues of which it is always continuous (23). To this the term hypapophysis strictly applies. The cervical vertebrae in many extant lizards have a ventral spur of identic origin, but together with this there is frequently present another element, intercalated ventrally between the vertebral centra, originating independently of these, though later it not unusually coalesces with the genuine hypapophysis, commonly of the posterior of the pair of vertebrae between which the primitively separate piece lies. Instances of such intercalated pieces are common. They are shown in the two annexed sketches of cervical vertebrae of Iguana sp. and Trachyosaurus |