30 SYSTEMATIC PALEONTOLOGY 



Family PHYSETERIDAE. 



Teeth entirely absent in the upper jaw and quite variable in the 

 mandible, either numerous as in the sub-family Physetehinae, or 

 reduced even to a single tooth in each jaw as in some of the Ziphiinae. 

 In both of these families the bones of the cranium posterior to the 

 nares form an elevated crest that converts the facial region into a 

 considerable concavity, it is in this concavity that the oily material 

 from which spermaceti is refined finds lodgement. Physeter, Meso- 

 plodon and Hyperoodon are living members of this family. 



Genus HYPOCETUS Lydekker.' 

 " The genus Paracetus has recently been proposed by Lydekker ' to 

 include members of this family (Physeteeidae) which possess a well 

 developed series of teeth in the (?) premaxillary and maxillary bones. 

 It is up to the present time represented by one species, the Paracetus 

 pouchetii Moreno, of the Santa Cruz beds of eastern Patagonia, of the 

 district of Chebut. The present species is apparently not distinctly 

 related to that one. . . ." Cope, Proc. Amer. Philos. Soc, pp. 135-155, 1895. 



Hypocetus mediatlanticus (Cope). 

 Plate XVII, Figs. 6a, 6b. 



Paracetus mediatlanticus Cope, 1895, Proc. Amer. Philos. Soc, voL xxxiv, p. 270. 

 Hypocetus mediatlanticus Hay, 1902, Bull. 179, U. S. Geol. Survey, p. .596. 



Description. — '* Char. Specif. As the posterior border of the skull 

 and the extremity of the muzzle of the specimen are broken off, an 

 exact idea of its outline cannot be given. However, the form was 

 probably much as in the P. pouchetii, and more elongate than in the 

 species Cogia. This form is subtriangular, with the base border con- 

 vex, and the two lateral ones concave. The muzzle is probably, how- 

 ever, produced into a rostrum, as the maxillary borders are parallel at 

 the point where it is broken off. On the right side, where the maxil- 



'This genus is called Hypocetus in his Catalogue of the Fossil Vertebrata of North 

 America. 



^Anales del Museo de la Plata; Paleontologia Argentina; II, Cetacean Skulls 

 frona Patagonia, p. 8, pi. iii. 



