OSTEOLOGY OF E. DOLLOVIANUS, COPE I 3 



zygapophyses in as well as up. The bottom line of the centrum 

 in the cervicals and dorsals has a prominent and very thin keel, 

 while this is absent in the posterior lumbars and the sacrals. 

 All these are characters which have been used to define differ- 

 ent genera and species, and their union in one form goes to 

 show how close are the different genera of the family Clepsy- 

 dropsidae and how difificult it is to identify forms from isolated 

 vertebrae. 



The atlas. — The first cervical is represented by a centrum only ; 

 there was probably a very rudimentary and loosely attached 

 neural arch which has disappeared. The first intercentrum, 



Fig. 10. — a, second intercentrum from below; b, same from the anterior face; c 

 an intercentrum from the posterior portion of the dorsal series. Natural size. 



between the atlas and condyle, is not preserved. That such an 

 element did exist is shown by its presence in Dimetrodon (i), 

 PI. II, Fig. 20. The second intercentrum, between atlas and 

 axis, is large and presents a deeply concave face for the posterior 

 face of the atlas. On either side of the intercentrum are 

 strong posteriorly projecting processes for articulation with the 

 cervical rib. In the collection of the University of Chicago there 

 are specimens of the atlas of Pelycosauria with the pre-atlas inter 

 centrum strongly attached to the atlas by co-ossification (Fig. 

 10 a and b'). 



MEASUREMENTS OF THE SECOND INTERCENTRUM. 



0.027 ' 

 0.012 



Greatest breadth -------- 



Antero-posterior length on mid-line of lower surface 



T\\& axis (Fig. ii) has a strong neural spine, with consider- 

 able antero-posterior extent ; in the last respect it differs quite 

 markedly from D'uiictrodon where the spine is rather thick and 

 not expanded in the antero-posterior diameter to such an extent, 



