27 



tion appeared some months before that of Mr. Couthouy, the 

 name of the shell is now Leda thracicEformis. 



Prof. Wyman exhibited to the Society the cranium of 

 a Hippopotamus recently presented, for the purpose of 

 alluding to some interesting points in the structure of the 

 teeth. On the true molars, which had been used, the enamel 

 was shown to be entirely exposed, while in one, not as yet 

 entirely through the gum, it was covered with crusta 

 jpetroso. Prof. Owen has ascertained that in nearly all 

 teeth, especially of Pachyderms, at a certain stage, the 

 crusta petrosa covers the whole surface of the crown, so 

 that the teeth of herbivora and carnivora may be referred to 

 the same general type, being composed of dentine, enamel, 

 and crusta petrosa, in regular succession. Prof. Wyman 

 showed by figures on the black-board, that the teeth of 

 Mastodons and Elephants are referable to the same general 

 type, each having its layer of dentine, enamel, and crusta 

 peirosa, the latter being quite thin in the Mastodon, and 

 early worn away. 



The canine teeth of the hippopotamus were shown to 

 resemble very clearly, both in form and anatomical elements, 

 the incisor teeth of Rodents. The lower, however, overlap 

 the upper, and the enamel is deposited on the front of the 

 lower and on the back of the upper. In the process of 

 mastication the opposed surfaces of these teeth are worn 

 away obliquely, the enamel forming the cutting edge. The 

 pulp in the cavity at the bottom of each tooth ref)roduces 

 it as fast as it is worn off. The thick coating of enamel 

 described by Prof. Owen as covering the upper surface 

 of the lower incisors where they come in contact with the 

 upper did not exist in the specimen exhibited. 



Dr. Gould said that he had recently received a number 

 of specimens of shells from California. He remarked, that 

 they were particularly interesting as showing differences 

 in species closely allied to each other on the east and west 



