1895.] l^J [Cope. 



Measurements. mm. 



Width of rostrum at end of specimen 35 



Width of lateral edge of right maxillary at inferior maxil- 

 lary foramen 25 



Length of inferior keel of vomer 120 



This species exhibits the most unsymmetrical cetacean cranium known 

 to me. Its size was probably about that of the ChonezipMus indicus of the 

 present ocean. Its exact locality is unknown, but it probably was ex- 

 posed to the action of water for a considerable time, after being washed 

 from its position of deposit. It has been bored by Pholades in several 

 places. 



BAL^NID^. 



I have remarked that the Mysticete with its single family the Balse- 

 nidffi * " would seem to have derived their descent from some form allied 

 to the Squalodontidse, since their nasal bones are more elongated than 

 those of the Odontoceti, and in Plesiocetus " (Celotherium) "the supe- 

 rior cranial bones show some of the elongation of that family." This 

 elongation of the superior crai^ial wall is not seen in the genus Squalodon, 

 but is moderately developed in the genus Prosqualodon of Lydekker, 

 founded on the P. australis Lydd. (I c.) from Patagonia. It is exhibited 

 in a still more marked degree by the genus Agorophius g. n. Cope, which 

 is represented by the Zeujlodon pygmceus of Miiller, which was referred 

 to Squalodon by Leidy.f The form of the skull in this genus approaches 

 distinctly that of Cetotherium of the Balsenidfe, and the permanent loss 

 of the teeth would probably render it necessary to refer it to the Mysta- 

 cocete. 



Stages of transition from, some such genus as Agorophius to the typical 

 whalebone whales are represented by several genera from the Yorktown 

 beds. Theoretically the loss of teeth bp failure to develop would be ac- 

 companied by the loss of the interalveolar walls, leaving the dental groove 

 continuous and separate from the dental canal. A genus displaying these 

 characters has not been discovered, but I have no doubt that it will be. 

 The new genus Siphonocetus Cope exhibits the groove roofed over by 

 ossification of the gum, and distinct from the dental canal. The genus 

 Ulias indicates that a still farther degeneracy took place, in the fusion of 

 the dental groove and dental canal, while the groove remained open. In 

 Tretulias the same condition persists with the addition that the gingival 

 passages and foramina are present, as in the genus Siphonocetus, and in 



*"0n the Cetacea," American Naturalist, 1890, p. 611 ; Meyer, Studien iiber Sdugethiere, 

 Jena, 1886, p. 191. 



t Extinct Mavim. Dakota and Nebraska, 1869, p. 420, PL xix, Fig. 8. It is doubtful 

 ■whether this geuus should be referred to the Zeuglodontidae or the Squalodontidse. TJie 

 relations of the maxillary and premaxillary boues posteriorly resemble most those of 

 the latter family. It differs from Prosqualodon in the contact of the temporal fossae on 

 the middle line above, and in the greater elongation of the frontal and probably nasal 

 bones. 



