442 Bulletin American Museum of Natural History. [Vol. XIX, 



part of the column present is 143 mm. No. 45146 (3766) 

 (PI. XXXVII, Figs. 3, 4) presents the head and 45 mm. of the 

 vertebral column. It has belonged to a somewhat smaller 

 specimen than the other. The striking feature of the verte- 

 bral axis of this form is that all the vertebree, from the head 

 to the tail, have been represented each by two rings or 

 "bodies," apparently just such a condition as we find in the 

 middle of the caudal region of Amia among living fishes and 

 in that of Eurycormus of the Jurassic. That this condition 

 exists in the species before us is shown by the fact that 

 throughout the series only alternate vertebral centra possess 

 neural spines, while in the caudal region only those centra 

 bear hasmal spines which have neural spines. 



In view of my results in the study of the vertebral column 

 of Amia (Field Columbian Mus. Pubs., Zool. Ser., I, p. 37, 

 1895), I hold that a vertebral ring possessing a neural arch 

 and one without such an arch together constitute the equiva- 

 lent of such a vertebra as we find in ordinary fishes ; and that 

 the archless ring belongs, not with the ring situated imme- 

 diately in front of it, but with the one just behind it. In the 

 species before us the rings, or "bodies," which have no neural 

 spines appear to possess neural arches, and thus seem to differ 

 from the corresponding elements in Amia. However, in the 

 tail of Amia the rings which have no neural arches have their 

 upper halves formed from ossifications which develop on the 

 upper surface of the notochord and on each side of the mye- 

 lon; and there appears to be no reason why these ossifica- 

 tions should not sometimes grow upward and form an arch 

 over the myelon. Such an arch would not, however, be the 

 equivalent of the arch which develops the spine, since this 

 arch is formed, in Amia at least, by a pair of bones distinct 

 from the centrum on which it rests. 



For reasons detailed in the paper referred to, I shall call 

 the 'centrum' which is devoid of either neural spine or 

 haemal arch an epihypocentrum ; that provided with either or 

 both arches, a pleurohasmacentrum. The former corresponds 

 with what Dr. Zittel and others call the hypocentrum; the 

 latter with what is called a pleurocentrum. 



