42 2 EXTINCT MAMMALIA OF NORTH AMERICA. 



The bottom is deeply concave, and, though more extensive than in the Porpoise, is 

 constituted in the same manner by portions of the parietals, frontal, and squamosal. 

 Posteriorly they are bounded by a prominent crest separating them from the inion. 

 Anteriorly they are also bounded by a prominent crest, formed by the frontal. 



Tlie vertex or top of the skull forms a short plane between the inion and forehead, 

 constituted by the conjoined parietals. On each side, the plane of the vertex forms 

 with the top of the temporal fossa a rounded margin, from which the parietal des- 

 cends into the fossa to meet the squamosal, the alisphenoid at bottom, and the frontal 

 in advance. 



The forehead, as composed by the frontals, continues the plane of the vertex for- 

 ward, and narrows outwardly in a ridge forming the anterior boundary of the tempo- 

 ral fossa. In the fossil, the fore part of the frontals where they conjoin the nasals 

 and ethmoid, together with these, is broken away. In the median suture the front- 

 als are separated by a deep elliptical pit, with sutured sides, which probably 

 accommodated a supernumerary bone. 



The supra-orbital portion of the frontals, with its post-orbital process, presents very 

 much the same form and relations as in the Porpoise. The ant-orbital portion of 

 the frontals is lost in the fossil. 



The basi-occipital and basi-sphenoid are co-ossified in the specimen. They are 

 proportionately narrow and very thick in comparison with their condition in the 

 Porpoise. As in the latter, the basi-occipital on each side extends downward and 

 outward into a large alary process, which, together with the paramastoid posteriorly, 

 and the mastoid externally, as in the Dolphins, bounds a large, though comparatively 

 wider and shallower recess, to accommodate the petro-tympanic bone. 



The zygomatic process of the temporal, in accordance with the comparatively 

 powerful mandible which articulated with its glenoid recess, is very much more robust 

 than in the Porpoise, though nearly like it in form. The glenoid articulation is a 

 broad concave surface, bounded behind by a strong post-glenoid tubercle. 



The mastoid process is far better developed than in the Porpoise, and projects 

 nearly as much as the paramastoid. Between it and the post-glenoid tubercle a 

 narrow arch is prolonged inward, corresponding with the position of the auditory 

 passage. A large rugged recess occupies the back part of the zygomatic root, extend- 

 ing below on the mastoid process. 



A large jugular foramen, and that for the hypoglossal nerve, occupy the same 

 relative position as in the Dolphins. 



The pre-, ali-, and orbito-sphenoids, together with the pterygoids, palatines, and 

 vomer, are all lost in the fossil. 



The maxillae and intermaxillEe are very Dolphin-like in their forms and I'elations, 

 but, as in other parts of the skull, diflfer strikingly in their great proportionate 

 strength. 



