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 ^emi-centroid (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 ex- 

 cellent 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 interiorly 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. Eurther, this author identified it with the foremost of 

 the " subvertebral wedge-bones " which in Ichthyosaurus supple- 

 ments 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 auto- 

 genous 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 hypa- 

 pophysis, commonly of the posterior of the pair of vertebrae between 

 which the primitively separate piece lies. Instances of such inter- 

 calated pieces are common. They are shown in the two annexed 

 sketches of cervical vertebrae of Iguana sp. and Trachyosaurus 



