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Show 1892.] AND OTHER CETACEAN REMAINS. 563 The portion of the cranium is represented from the upper surface in PI. X X X V I I I . fig. 2, and, although somewhat crushed, is in a fair state of preservation. Alongside the Caucasian specimen there is represented (fig. 1) the corresponding portion of the cranium of an extinct Cetacean from the Tertiary of Argentina, described by Burmeister1 under the name of Pontistes rectifrons (Bravard), which belongs to the Platanistidee. A comparison of the figures will show the close relationship of the two forms at a glance. This is especially manifested 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 fossae are characterized by their depth and their squared posterior borders, which run close up to the parieto-occipital surface. On the other hand, in the Delphinidee 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. X X X V I I . figs. 3, 3 a 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 Schizodelphis sulcatus3, 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 Schizodelphis, that genus (although placed among the Platanistidee) 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 1 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 argen- 1 Ann. Mus. Buenos Ayres, vol. iii. p. 138, pi. ii. (1885). 2 The name Pontoporia being preoccupied, it is necessary to adopt tbe later Stenodelphis. 3 Gervais, Zool. et Pal. Franchises, 2nd ed. pi. Ixxxiii. 4 Sci. An. Mur. Buenos Ayres, vol. iii. p. 451 (1891). |