246 



ox THE IXTERCENTRUM OF THE 



to the genns Cricotus, type of the order Embolomeri ; a form which has not yet been 

 found in Europe, so far as ascertainable. lu this genus the chevron bones are con- 

 tinua, while the neural arch is free, as in Eryops. The element which bears the 

 chevi'ou l)one being regarded, as in that genus, as the intercentrum, we can affirm 

 (Fig. 3) that the intercentrum is continued upwards to the neural canal, forming a 

 disk, and the pleui-ocentra are continued downwards and are united below, forming a 

 complete centrum. This explanation looks the more reasonable in view of the exist- 

 ence of the hy[)oeentrum ])leurale in the Sphenosauridjc, which if combined with the 

 pleurocentra would foi'm a completed centrum. It is, however, uncertain as yet 

 whether this is the make-up of the centrum in Cricotus, because in Eryops the hypo- 

 centrum pleurale unites with the intercentrum and not with the pleurocentrum. That 

 my determination of the homologies of the vertebral disks in Cricotus is correct is 

 further evidenced by the structure of the caudal region of Eryops. In E. erycholiticus 

 (PI. I, Fig. 1), the pleurocentra descend further than in the dorsal region, reaching 

 to the inferior face of the column, and sciJai'ating the intercentra from mutual contact. 



Fig. 3. — Cricotus crassidiscua Co[)c, posterior part of vertebral column two-fifllis natural size; original; from 



Amerifan Naturalist, 1884, p, 1)7. Fig. n, proatla.s ; h, ilo., vvilli first cervical ; e, cervicals ; d, proximal caudals 



from below ; e, distal caudals left side. I'li, pubis ; U, ilium ; in, iscliium ; i, intercentrum ; ce, centrum (pleuroccM- 

 trum) ; r, ribs. 



Fiu-ther development of their inferior portion, with truncation of inferior surface, 

 would represent the structure stated by Fritsch to characterize Archegosaurus, and 

 which is still better developed in Cricotus. The chevron bones are always continua 

 ill l)()lli Eryops and Cricotus. Of the latter genus I possess in my collection many 

 c;iu(l;il intercentra with chevrons, of all ages, and in none of them is there any sutural 

 articulation visible (I'l. I, Fig. G). 



The correctness of this determination is further confirmed by a study of the dorsal 

 region of Cricotus. In passing along the caudal region towards the sacrum, the 

 chevron-bearing disks or intei-centra diminish in anteroposterior di;imeter both abso- 

 lutely and i-elatively, while the centi'a do not un<lei-go any modification. The neural 



