176 PROCEEDINGS OP THE ACADEMY OP 



CHALICOMYS, Kaup, 1832. 



Aulacodus, Chelodus, Kaup, 1832 : Sleneofiber, Stcneotherium, St. Hil. 1833 ; 

 Castor, Lin. : Kaup, Gervaise. 



1. ChALICOMYS NEBRASCENSIS. 



Steneofiber nebrascensis, Leidy : Pr. A. N. S. viii. 89 ; ix. 89. 

 DREPANODON, Nesti, 182C. 



3Ieganiereon, Croiz, 1828; Agnotherium, 3Iachairodus, Kaup, 1833; Steneodon, 

 Oroiz, 1833; Smilodon, Luad, 1841; Ursus, L., Felis, L. : Guv., Croiz, Brav., 

 Blainv,, &c. 



1. Drepanodon primaevus. 

 Machairodus primaevus, Leidy and Owen : Anc. Fauna of Nebraska, 95 ; Pr. 

 A. N. S. ix. 90. 



DORCATHEPJUJM, Kaup, 1833. 

 Leptomeryx, Leidy. 



1. DOECATHERIUM EvANSI. 



Leptomeryx Evansi, Leidy : Proc. A. N. S. vi. 394 ; ix. 89. Lcpiomeryx is per- 

 haps at most only subgenerically distinct from Dorcatherium. Some fragments 

 of jaws with teeth, obtained by Dr. F. V. Hayden, prove that the large tubercle, 

 which is described as rising out of the base internally of the upper true molars, 

 in a specimen of the skull of the same animal, is an inconstant character, and 

 independent of this, the molar teeth of Leptomeryx and Dorcatherium are identi- 

 cal in form. 



Seplemher 1st, 1857. 

 Vice-President Bridges in the Chair. 



Dr. Leidy remarked that there appeared to exist a general misconception in 

 regard to the dentition of the 3Iososaurus. The animal is almost universally 

 called an acrodont reptile, or one in which the teeth are inserted upon, or are 

 co-ossified with, the border of the jaws. A number of specimen^ of teeth and 

 fragments of jaws, in the museum of the Academy, prove this appellation to be 

 incorrect. 



The teeth oi Mososauriis have a recurved pyramidal crown, and a more massive, 

 vertically oblong root, which is often twice the length of the crown. The root 

 is inserted for three-fourths of its extent into a correspondingly deep socket, witli 

 the sides of which it is co-ossified. The centre of the teeth is occupied by a 

 fusiform pulp cavity, communicating with one or more vascular canals passing 

 through the fang. 



In the reproduction of the teeth, it appears the new ones commence to be 

 developed attached to the gum, on the postero-internal side of the alveoli. As 

 they proceed, they penetrate into the latter, by exciting an absorption of the 

 substance of the fang of the old teeth in a direction obliquely outward and for- 

 ward. The cavity for the new tooth increases in size at the expense of the 

 substance of the fang of the old one. The pulp cavity of the old tooth, in con- 

 sequence of the ossification of its pulp, appears to recede before the increasing 

 cavity of the new tooth. The latter cavity finally makes a communication with 

 the former, though this .appears not always to be the case ; and subsequently 

 the fang of the old tooth becomes so completely excavated as to form a mere 

 capsule, from which its crown is brolcen away, or shed, through comparatively 

 little violence. In the further progress of the newly protruding tooth, the 

 osseous capsule formed from the fang of the old tooth is gradually obliterated, 

 except a portion which remains as a partition frona the next alveolus. 



[September, 



