.MKIiHIAM: TRIASSIC ICIITIIYOSATKIA. 



31 



a Triassir vertebra probably belonging to an ichthyosaurian. Somewhat sim- 

 ilar perforations appear occasionally in some of the later ichthyosaurs as in 

 Ichthyosaurus <-ftt fit/lotion." In most forms the character of the excava- 

 tion is similar to that in the typical species of lcJttJii/<txnn>-Hx (fig. 17), the 

 concavity beginning near the periphery of the centrum 'and sloping evenly to- 

 ward the center. In Cymbospondylus petrinus (fig. 21), the best known Mid- 

 dle Triassic form, the outer portion of the articular face is nearly flat or only 

 slightly hollowed, and the excavation cuts in sharply at about half of the dis- 

 tance to the middle. The flattening of the peripheral portion of the terminal 

 faces is very pronounced in the posterior dorsal region. 



21 ' 22 23 24 25 



Figs. L'l-25. Cymbospondytu* in'trhnix Lekly. Outlines of vertebrae, X %. Fig. 21, anteroposte- 

 rior cross-section of a dorsal centrum; fig. 22, anteroposterior profile of 3rd centrum; fig. 23, 

 anteroposterior profile of anterior dorsal centrum; fig. 24, anteroposterior profile of posterior 

 dorsal centrum; fig. 25, anteroposterior profile of posterior caudal centrum. 



The occurrence of vertebrae in which the terminal faces are largely filled 

 out in some of the earliest known ichthyosaurs naturally raises a question as to 

 whether the deeply biconcave centra of the Upper Triassic and later genera are 

 primitive, or whether they have been secondarily developed as a part of the 

 adaptation to aquatic life. The more nearly solid centrum is evidently in gen- 

 eral the type best fitted to meet the requirements in active land or shore forms. 

 It has appeared in many of the reptilian groups, and even in certain of the more 

 highly specialized Stegocephalia. Its development is possibly to some extent a 

 function of size in the individual among land forms, as is indicated by its ap- 

 pearance in the large "Stegocephalia and many reptiles ; the deeply biconcave 

 vertebrae persisting principally in the small land reptiles. In aquatic forms 

 in which the vertebral column is relieved of a part of the strain of supporting 

 the body the biconcave type of centrum may prove adequate. Of the speci- 

 mens of the earliest European ichthyosaurs examined by the writer none have, 

 however, been found to show such a degree of marginal flattening as is seen in 



is Kiprijanoff, W., Mem. L'Acad. Imp. des Sc., St. Petersb. 7 ser., t. 28, no. 8, p. 77. 

 See also Dames, W., Zeit. d. deutsch. geol. Gesel. 1893 Bd. 45, p. 30. 



