364 U. S. P. R. R. EXP. AND SURVEYS ZOOLOGY GENERAL REPORT. 



rior angle of the orbit ; as already stated, the malar does not extend as far as this bone. The 

 ante-orbital tubercle of the frontal, just posterior to the lachrymal, seems wanting in Castoroides, 

 and the greatest contraction of the skull is across the parietal, not the frontal bones, as in 

 Castor. This contraction, too, is decidedly behind the middle of the skull, instead of being 

 considerably anterior to this point. 



Owing to the more elevated position of the zygoma, the exterior inferior outline is much 

 longer than the extero-anterior, and is nearly parallel with the crowns of the molars, instead of 

 having the two lines just mentioned of the same length and the former one quite oblique. 



The incisive foramina are very inconspicuous and short, nearer the molars than in Castor. 

 The palate, however, is similarly furrowed. The palato-maxillary foramina are also small, and 

 not so far back as the junction of the second and third molars. The posterior margin of the palate 

 is slightly concave, instead of running out in an azygos process, and the emarginate notch passes 

 forward, so as to separate a little the posterior molars. The inner wing of the pterygoid pro 

 cesses is bent inwards, so as to come into contact, by the convexity of the fold, with its fellow. 

 The curious sub-quadrate pit or depression of the under surface of the occipital bone of Castor 

 is not found in Castoroides ; the auditory bullae are much less. 



The axial lines of the upper molars are much more convergent than in Castor ; and are longer 

 in proportion, occupying more than one-fourth of the length of the skull ; posteriorly this line 

 reaches within its own length of the occiput, while in Castor it falls short by more than one and 

 a half lengths. 



The pattern of the grinding surface of the molars is strikingly different from that of Castor ; 

 instead of being formed by the indentation of longitudinal folds of one exterior enamel tube, 

 one on the inside and three on the outside in the upper jaw, there are in each tooth three closed 

 transverse loops of enamel, completely separated from each other by cement ; the posterior upper 

 and the anterior lower molars having four. The incisors on their antero-external faces exhibit 

 a number of broad shallow grooves. Their projection anteriorly seems much greater than in 

 Castor. These teeth are much larger and stouter in every way, being three times as thick as 

 those in a skull of Castor, half the length of Castoroides. 



The length of the skull of Castoroides figured by Dr. Wyman, exceeds nine inches from 

 the occiput to the end of the intermaxillary ; its greatest height is four inches; greatest breadth 

 between zygomata seven inches. The length of the skull of Castor which served for the pre 

 ceding comparisons, is five inches; height, two and a quarter; width, three and three quarters. 

 Castoroides had, therefore, a skull about twice the linear dimensions of Castor, but much more 

 than twice as massive, judging from the incisors ; its bulk may be reasonably assumed as six 

 times that of the large beavers, which are known frequently to weigh 40 pounds ; this would 

 give 240 pounds for the weight of Castoroides. 



The specimen of Professors Hall and Wyman was obtained in Clyde, New York, in a swamp 

 on the summit level between the waters of Lake Ontario on the north, and Cayuga and Seneca 

 lakes on the south. It was found about eight feet below the surface in a muck bed about four 

 feet thick, resting on sand with shells of Planorbis, Cyclas, &c., and separated from a similar 

 layer on the surface by a stratum of fine sand with occasional seams of clay; according to Pro 

 fessor Hall, it was found in a lacustrine formation subsequent to the drift. 



There is still another fossil genus, Trogontherium, allied to the beaver and still more to Cas 

 toroides, which it closely resembles in the character of the teeth. This is found in peat bogs of 

 Europe, and was about one fifth larger than the beaver. Like Castoroides, it occurs in lacus 

 trine formations over the drift. 



