1892.] AND OTHER CETACEAN REMAINS. 563 



The portion of the cranium is represented from the upper surface 

 in PI. XXXVIII. fig. 2, and, although somewhat crushed, is in a fair 

 state of preservation. Alongside the Caucasian specimen there is re- 

 presented (fig. 1) the corresponding portion of the cranium of an ex- 

 tinct Cetacean from the Tertiary of Argentina, d escribed by Burmeister '^ 

 under the name of Pontistes rectifrons (Bravard), which belongs to 

 the Platanistidce. A comparison of the figures will show the close 

 relationship of the two forms at a glance. This is especially mani- 

 fested by the form of the maxillary fossae lying on the two sides of 

 the narial aperture. In both the fossils, as well as in the existing 

 Stenodelphis " and Inia, these fossse are characterized by their depth 

 and their squared posterior borders, which run close up to the 

 parieto-occipilal surface. On the other hand, in the Delphinidce 

 these fossae are shallower, and shelve upwards towards the occiput, 

 where they terminate gradually in a curved border. 



The fragment of jaw represented in PI. XXXVII. figs. 3, 3 c is one 

 of a pair, and is, I think, a portion of the rostral region of the left 

 maxilla. It contains 13 dental alveoli in the space of 5 inches, and 

 is of a long and slender form. The alveoli are laterally compressed. 

 These jaws have a great resemblance to those of the European 

 Miocene SchizodelpMs sulcatus ^, in which the dental alveoli have 

 the same compressed form. If, however, as I think probable, they 

 belong to the form under consideration, it is quite evident that they 

 cannot be referred to SchizodelpMs, that genus (although placed 

 among the Platanistidce) having the maxillary fossae of the ordinary 

 Dolphin-like form. 



The vertebrae do not call for any notice beyond the bare mention 

 that the centrum of a lumbar measures 1"3 inch in length and I 

 inch in width. 



Compared with Inia, the Caucasian cranium differs in the 

 absence of the high prominence behind the nares, in the smaller 

 development of the ridges bounding the maxillary fossae, and in the 

 less-inclined occiput. Assuming that the lower jaw belongs to the 

 same form, the teeth will also differ in their relatively smaller size 

 and lateral compression. 



In many respects the fossil skull is more like Stenodelphis, but 

 the maxillary fossae are deeper, with more prominent borders ; while 

 the recent form has not the large and well-defined square surface 

 behind the nares between these fossae. The teeth of Stenodelphis 

 are, moreover, cylindrical. 



In Pontistes the occipital surface is more inclined forwards than 

 in the Caucasian fossil, and the space between the fossae behind the 

 nares is also narrower. The dental alveoli are, however, elliptical 

 in both. A much larger form has been described by Burmeister * 

 from the Argentine Tertiary under the name of Saurodelphis argeii- 



1 Ann. Mus. Buenos Ayres, vol. iii. p. 138, pi. ii. (1885). 

 ^ The name Pontoporia being preoccupied, it is necessary to adopt the later 

 Stenodel2:)his. 

 ^ Gervais, Zool. et Pal. Frangaises, 2nd ed. pi. Ixxxiii. 

 * Sci. An. Mur. Buenos Ayres, vol. iii. p. 4.51 (1891). 



