THE VERTEBRAE 



lOI 



Fig. 78. 



Ophiacodon. Proatlas, axis, 

 and ribs. 



habits. The marine crocodile, with a fin-Hke tail, lost two, like the 

 mosasaurs and aigialosaurs, having seven; Pleurosaums probably 

 had but five ; and the ichthyosaurs, the 

 most specialized of all aquatic reptiles, 

 had practically no neck. 



The first two or three of the cervical 

 vertebrae are markedly differentiated 

 in all reptiles, as in the higher animals. 

 The first of these, the proatlas, is in- 

 constant and vestigial, and has not 

 been included in the numbers above 

 given. The second, the first of our 

 usual nomenclature, is the atlas. The 

 third, more or less closely united with 

 the atlas, is the axis, or epistropheus. 

 The following cervical vertebrae, when 

 present, are differentiated more or less 

 from the dorsal series by their less 

 erect or shorter spines, transverse pro- 

 cesses, or the slenderness and mode of 

 rib articulation. The cervicals of the 

 later pterodactyls have additional ar- 

 ticulations on their ventral sides, as 

 has been described above (p. 91). 



Proatlas. The proatlas (Figs. 79 c, 

 80 D, l) is a small, more or less vesti- 

 gial neural arch between the arch of 

 the atlas and the occiput, usually 

 paired. It is believed to be the arch 

 of a vertebra formerly intercalated be- 

 tween the atlas and the skull; by 

 some, homologous with the so-called 

 atlas of the Amphibia; by Baur, as the 

 representative of a vertebra fused with 



the occiput in the reptiles; by others, as merely the separated spine 

 of the atlas; by others, as the arch of a vertebra whose centrum is 

 represented by the anterior end of the odontoid. Another theory, 

 which has less to commend it, is that of Jaekel, namely, that the 



Fig. 79. Theromorph vertebrae: A, 

 Dimetrodon, atlas and axis; B, the 

 same atlas, from the front; C, the 

 same proatlas, from the side; D, 

 Sphenacodon, neurocentrum of atlas, 

 inner side. /, intercentrum; o, pleu- 

 rocentrum (odontoid); «, neurocen- 

 trum (arch). 



