EXTINCT MAMMALIA OF NORTH AMERICA. 421 



glodon, or BasiIosauru.i, atid his description and figure, repeated by Mliller, constitute 

 the basis of liis ZeugJodon pygmceus. 



Prof. Holmes, to whom the specimen now belongs, has recently submitted it to my 

 examination. It indicates a species intermediate in size to Sqnahdon utlanticus and 

 S. pelagiiis. It is represented in figures 7, 8, plate XXIX, reduced one-half. 



In general form the skull of S. pygmceus is much like that of the European species 

 Squalodon EhrUchii, as represented by Van Beneden in his " Recherches sur les Squa- 

 lodons, Bruxelles, 1865, Plate II." 



The skull is intermediate in form with that of the living Dolphins and tliat of 

 Dorudon, as represented in the nearly complete skull represented in plate XXVI, of 

 Miillers work on the Zeuglodonts, under the name of Zenglodon hrachyspondylus* 



The skull of Squalodon pygmrrus, in comparison with that of the Porpoise, exhibits 

 a proportionately greater degree of compactness and contraction, much greater 

 strength and decidedly a more carnivorous aspect. The cranium is less voluminous, 

 of much less capacity as concerns the cerebral cavity, is depressed behind instead of 

 being protuberant, and is strongly constricted at the middle. The bones of the skull 

 of the living Dolphins may be said to be comparatively light and laminar ; those of 

 Squcdodon pygmaius are decidedly ponderous. The cranial axis in the former is com- 

 paratively thin, in the latter it is proportionately several times as thick. Everything 

 indeed in the skull of Squalodon would appear to indicate the power of contending 

 with and securing stronger prey than the modern Dolphins are able to do. 



The inion of Squalodon pygmcBus is proportionately not so great as in the Porpoise 

 and is depressed or moderately concave instead of being decidedly convex. Its sum- 

 mit relatively does not reach so far forward, but is separated by a comparatively long 

 interval from the forehead, though relatively not to the same extent as in Basilo- 

 saurus. 



The temporal fosste are of greater capacity than in the Dolphins, and approach 

 within a fourth of the distance at their upper part, though this is much less than 

 in Dorudon, in which they are only separated by a sagittal crest. 



The outline of the inion, formed entirely by the occipital, has the form of a longi- 

 tudinal section of a blunt cone. The surface is moderately concave, the supra-occipi- 

 tal portion being bent forward. The ex-occipitals present a posterior vertical plane 

 ending in a broad, strong paramastoid process. The condyles, nearly like those in 

 the Porpoise, project from the lower part of the inion by a more abrupt and con- 

 stricted neck than in the latter. The occipital foramen is transversely oval. 



The temporal fossae are much wider fore and aft than from above downward, being 

 the reverse of tlie condition in the Porpoise. They are separated at the vertex by an 

 interval between the inion and the forehead, about an inch and three-fourths wide. 



*Uebcr d. fos. Reste d. Zeuglodonten v. Nordamerica, Berlin, 1849. 



