139 



reference to the insertion of the muscles of mastication. The 

 molares consist of lamincC, the first lower and the last upper 

 tooth each having four, the rest only three ; which, as in the 

 elephant and Hydrochcerus, consist of dentine enveloped with 

 enamel, and are united together by csementum ; the number of 

 lamince, however, in the posterior molares of the Hydrochserus 

 amounts to thirteen, and the interspaces are imperfectly filled with 

 csementum, so that the edges are serrated, and in the interior 

 teeth the enamel is more or less involuted ; in the Castoroides 

 the number of laminae does not exceed four, and the csementum 

 fills the whole of the interspaces, so that the tooth is destitute of 

 serrated edges. 



The internal pterygoid processes, differ from those of all 

 existing genera, having a remarkable curvature inwards towards 

 the median line, the most prominent points coming in contact 

 on the median line, so as to divide the entrance to the posterior 

 nares into two. The internal pterygoid fossae are deep excava- 

 tions, as is the case in the Fiber, where the external as well as 

 the internal process is well developed. 



The jaw described by Mr. Foster somewhat exceeded in size 

 that described in the present article. 



The generic characters, which have been deduced from this 

 cranium, are as follow : 



Teeth : Incisors | -f, Canines % §, Molars | J, total 20 ; in- 

 cisors longitudinally striated in front. Molars consist of thin 

 laminae of dentine, surrounded by enamel, and the different 

 laminae united together by crusta petrosa ; the first tooth in the 

 upper, and the last in the lower jaw have each four laminae, and 

 the remaining teeth have only three each. The grinding sur- 

 faces of the crowns are nearly flat, but slightly excavated be- 

 tween the ridges of enamel. The internal pterygoid fossae are 

 largely developed, and the internal processes so far deflected 

 inwards as to meet on the median line, and divide the entrance 

 to the posterior nares transversely, thus forming a superior and 

 an inferior orifice. 



DONATIONS TO THE JIUSEUM. 



Tertiary Fossils from Acquia Creek,Virginia. From Professor 

 Jeffries Wyman. 



