336 ON THE EXTINCT MAMMALIA OF 



tinuation of the sagittal suture. A portion of the" bone on one side is broken away, 

 and the apparent suture may have been accidental. The forehead is broader than in 

 the Muskrat, and is ti'ansversely convex, but slightly depressed at the middle. The 

 posterior extremity of the frontal is received into a deep notch of the parietals. The 

 anterior border pursues nearly a similar course across the face as in the Beaver. 



The face appears to have been nearly as broad proportionately as in the Beaver. 

 The infra-orbital foramen appears to have been as large as in the Muskrat. 



The hard palate is wider and not so deep as in the latter animal. The incisive 

 foramina extended as far back as the maxillo-intermaxillary suture. The palate 

 plates of the palate bones together form an isosceles triangle reaching as far forward 

 as the position of the second molar teeth. 



The basi-occipital is much narrower than in the Muskrat, and, as in this, presents 

 a median keel. The basi-sphenoid is of more uniform breadth, and forms a narrow 

 inclined plane. 



The auditory bulla is large and oval. 



The fossil contains on one side all the molar teeth, five in number, and upon the 

 opposite side all except the last one. 



The fragments of lower jaws, attributed to the same species as the skull, consist of 

 alveolar portions, of which one contains a portion of an incisor, the second molar, the 

 socket and fangs of the first, and portions of the sockets for the third and fourth mo- 

 lars ; and the other specimens contain the second and third molars and portions of 

 the sockets of the first and fourth. 



The fossils would indicate the dental formula to consist of the usual number of in- 

 cisors, and five molars to each side of the uj^per jaw, and four to each side of the 

 lower jaw. 



The upper molars of the fossil, represented in figure 4, plate XXVI, three times 

 the diameter of nature, belong to the permanent set, and are constructed after the 

 plan of those of the Squirrel family. They have rounded cuboidal, tuberculated 

 crowns invested with thick enamel and devoid of obvious cementum, and they are 

 inserted by well-developed fangs. 



The lower molars bear a near resemblance to those generally of the upper jaw in a 

 reversed position. 



The first upper molar is the smallest of the series, and appears to be inserted by a 

 single fang. Its crown is barrel-shaped, with the triturating extremity impressed by 

 a crescentic fossa, which separates an antero-external conical eminence from an in- 

 ternal crescentic ridge. 



The succeeding three superior molars are the largest, and nearly of uniform size. 

 The fifth molar is slightly less than those immediately in advance. 



The back four upper molars bear some resemblance to those of the Marmot, Arcto- 



