NO. 1875 ^^'^V FOSSIL CETACEAN — TRUE 451 



and extending about 7 mm. beyond their inner surface. This third 

 branch is shorter than the two principal ones. The crown is com- 

 pressed and conical, but worn away at the apex, and also anteriorly. 

 It is rugose, like the lower molars, and presents bases of two large 

 denticles on the posterior edge. 



The smaller upper molariform tooth (pi. xliv, fig. 4) is diflferent 

 from any of the others in form. It is probably the last left upper 

 molar, or possibly a premolar. The root is triangular, broadest at 

 the base, very uneven, somewhat curved inward, convex externally, 

 and marked internally by a narrow longitudinal groove, representing 

 an incipient division into two branches. The neck is strongly marked 

 and very smooth. The crown, which is nearly complete, is thick, 

 conical, and very rugose. Beside the ordinary rugosities, there are 

 on the posterior edge the remains of five denticles, arranged in two 

 rows, and marking the boundaries of an elliptical area, which ter- 

 minated near the apex of the crown. This peculiarity is of much 

 interest, as a similar arrangement of denticles is found in various 

 genera belonging to families allied to the Squalodontidae. On the 

 anterior edge of the tooth are the bases of two similar denticles in a 

 single row. 



Tympanic Bulla 



The right tympanic bulla and periotic bone, which accompany the 

 jaw, appear at first sight too small to have belonged to the same indi- 

 vidual as the latter, but on 

 comparing them with Lortet's 

 figure of Squalodon hariensis, 

 a species of about the same size 

 as Prosqualodon australis, I 

 find that the bulla of the Pata- 

 gonian specimen is quite as 

 large, or even larger. It bears 

 a superficial resemblance in 



form to that of Schizodelphis, _ _ 



,,,,.. 1 • n 1 ,1 Fig. 77. — Tympanic bone of Prosqualo- 



but this IS chiefly because the , . ;• t ^ 1 1 



■' don australis Lydekk. 



anterior portion is broken off, inferior surface. Nat. size, 



leaving a sharp point. Orig- 

 inally the bulla was probably nearly as broad anteriorly as pos- 

 teriorly, and presented, therefore, much the same shape as that of 

 Squalodon. 



The bulla (fig. yy) is everywhere quite rugose. Viewed from the 

 inner side, the inferior outline is nearly straight, and the posterior 

 4 



