TRUE : THE FOSSIL CETACEAN, DORUDON SERRATUS. 73 



served is 27 mm., but there are sundry dark spots and rough points on 

 the roots which appear to indicate that the crown was originally some- 

 what deeper, perhaps as much as 40 mm. all together at the middle on 

 the inuer face and 36 mm. on the outer face. This would change the 

 shape of the crown materially and bring it much nearer the margin of 

 the maxillary bones than is indicated in Plate 1, figures 2 and 4. How- 

 ever it may be with this premolar, the crown of the molars is certainly 

 much less deep than in Basilosaurus. 



The second premolar has three accessory cusps both anteriorly and 

 posteriorly, the former smaller than the latter and further removed from 

 the apex of the principal cusp. The free border of the anterior cusp 

 adjoining the basal one bears a thin, sharp ridge, which is also apparent 

 on the cusp next above. The anterior and posterior edges of the princi- 

 pal cusp are sharp, the former most so. The internal and external sur- 

 faces of the crown are convex, and the enamel, although everywhere 

 somewhat rugose vertically, is conspicuously so only on the internal 

 surface near the base of the crown. The roots are hollow, as mentioned 

 by Gibbes. 



Whether the three teeth (Plate 1, figure 2) which form part of Gibbes's 

 types immediately follow the premolar tooth just described cannot be 

 positively determined, but it is probable that they do ; and they are, 

 therefore, the third and fourth premolars and the first molar. 



The third premolar is a somewhat smaller tooth than the second, with 

 the two roots less divergent. The crown is badly broken, only the 

 penultimate posterior accessory cusp remaining intact. Nearly all of 

 the internal half of the tooth is lacking, but its shape is indicated by the 

 green sand with which it was filled. The length of the posterior root 

 from its junction with the anterior root is 43 mm., but was originally 

 somewhat longer. The antero-posterior breadth of the crown is about 

 49 mm., and of the anterior root at its junction with the crown, 17 mm. 

 The transverse diameter of the root at the same point was originally 

 about 12 mm. The distance from the margin of the maxilla to the 

 crown at the middle is 11 mm. The small portion of the enamel which 

 remains is nearly smooth. The free margin of the penultimate posterior 

 accessory cusp bears a sharp, thin ridge. 



The fourth premolar follows the third with scarcely more than one or 

 two millimeters intervening. Its anterior root, however, is a little ex- 

 ternal to the posterior root of the third premolar, and the form of the 

 tooth is quite different. The anterior root at its junction with the crown 

 has an antero-posterior diameter of 21 mm. and a transverse diameter 



