362 CETACEANS. 



one inch, three lines), placed parallel with the axis of the jaw. 

 The isthmus is about three lines in breadth, and two in length ; 

 but the breadth diminishes while the length increases as the tooth 

 descends in the socket, until it finally disappears, and the two por- 

 tions take on the character of separate fangs. 



The mode of completion of the teeth in this extinct Cetacean is 

 different from and conforms to a higher type than that of any of the 

 existing carnivorous genera. It is evident that the pulp which, from 

 the form and structure of the crown, was originally simple, becomes 

 afterwards divided into two parts, and that its calcification then 

 proceeds towards two distinct centres, which are each separately 

 surrounded by concentric striae of growth. The cavitas pulpi, which 

 is very small in the crown of the tooth, becomes contracted as the 

 fangs descend, and is almost obliterated near their extremities. 



Of the fragment of the lower jaw of the Zeuglodon there is 

 a plaster cast in the Museum of the Geological Society. It contains 

 four teeth of which the two posterior are nearly contiguous ; the 

 next is separated from them by an interval of an inch and a-half, 

 and the most anterior is placed at a distance of two inches from 

 the preceding. The anterior tooth is here of smaller size, and 

 apparently of more simple form than those behind, and it is 

 described by Dr. Harlan as a canine. This interesting fragment 

 is preserved in the Museum of the Philadelphian Academy : it 

 confirms the evidence afforded by the fragments of the upper jaw, 

 viz: — that the teeth in the Zeuglodon were of two kinds, the 

 anterior being smaller, more simple in form and more remote from 

 each other, than those behind. 



The summits of the crown of the teeth of the Zeuglodon were 

 most probably sheathed with enamel ; their base exhibits an in- 

 vestment of a thin layer of cement which augments in thickness 

 where it surrounds the fangs. 



In a fine transverse section taken from below the middle of 

 the exposed crown, I found this cement presenting the same micro- 

 scopic characters as that in the Cetacea ; being traversed by nume- 

 rous transverse parallel tubuli. The Purkinjean cells are scattered 

 in some places irregularly, in others arranged in parallel rows; 



