1894. '^HE LA PLATA MUSEUM. 125 



was obtained from a sandy deposit a short distance from the 

 shore at Chubut. Most of the other remains are those of 

 toothed-whales, among which a fine, though somewhat imper- 

 fect, skull of a small form allied to the sperm-whale claims 

 special attention. As our readers are doubtless aware, the whole 

 of the existing members of the sperm-whale family are characterised 

 by the absence of functional teeth in the upper jaw, those in 

 the lower jaw varying in number from more than twenty on each 

 side to a single pair. The Patagonian skull shows, however, a full 

 series of large conical teeth in both the upper and lower jaws, 

 these teeth being not unlike those of the sperm-whale, although 

 furnished with thin caps of enamel on their crowns. The skull has 

 the same general form as that of the cachalot, displaying a large and 

 deep frontal cavity for spermaceti. From the structure of the teeth 

 1 have identified this skull with the European Tertiary genus Pliysodon, 

 which has hitherto been but very imperfectly known ; and since its 

 inclusion in the Physeteridae would render that group very difficult to 

 define, I have suggested that it should constitute a family by itself. 

 Another member of the same family is represented in the Museum by 

 a smaller cranium, to which I have assigned the name of Hypocetus. 

 A totally different type of Cetacean is presented by a small 

 skull v/ith teeth of the type of those of the European Tertiary 

 genus Squalodon, but differing from the latter in number. This 

 difference alone I should not have regarded as of generic value, 

 but an examination of the nasal region showed the presence of 

 prominent nasal bones projecting over the nasal cavity in a manner 

 quite unknown in any living member of the suborder, and I accor- 

 dingly consider this form as the representative of a new genus, with 

 the name of Pvosqualodon. Precisely the same feature, although in a 

 more exaggerated degree, is displayed in the nasal region of an 

 exceedingly elongated and dolphin-like skull, with simple teeth, 

 which I have described under the name of Argyrocetus. From its 

 general characters, I refer this skull to the Platanistidse, but it differs 

 from that of the three existing genera of that family by the symmetry 

 of the narial region, and the projecting, wedge-shaped, and roof-hke 

 nasal-bones. Although the discovery, sooner or later, of toothed- 

 whales with projecting nasals and symmetrical skulls was a thing to 

 be expected, yet the absence of any evidence of the existence of such 

 forms hitherto has been regarded as a bar to the derivation of the 

 baleen-whales from the toothed-whales. This obstacle has now been 

 removed by the discovery of these two extinct genera in the 

 Patagonian Tertiaries, and it is possible that future investigations 

 will show that certain other features, which have been regarded as 

 indicating a dual origin for the two groups in question, admit oi 

 another explanation. The inclusion of these two forms in 

 the Odontoceti (and they certainly cannot be regarded as repre- 

 senting a distinct group) must, to some extent, modify the ordinarily- 



