452 MONOGRAPHS OF NOKTIi AMERICAN BODBNTEA. 



This species, so far as these specimens indicate Its character, appears to 

 resemble the genus Steneqfiber, from the lower Pliocene formation of Saint- 

 Ge'rand-Ie-Puy, France, both in the general form of the skull and in its den- 

 tition. So great is this resemblance that Dr. Leidy at first referred it to 

 that genus, which Kaup has regarded as identical with Chalicomys. The lower 

 jaw in Steneqfiber, says Leidy, is unknown, and adds that that of Palieocastor 

 is quite unlike the lower jaw in Chalicomys. The structure of the molars in 

 Chalicomys differs greatly, as shown by Gervais's figures,* from that of Steneo- 

 Jiber, and Geoffroy, Gervais, and others regard the two forms as generically 

 distinct. In Pala-ocastor, the structure of the molars is very similar to what 

 is seen in the figures of the molars of Steneqfiber. In respect to the skull, 

 Leidy also observes that the "forehead presents the same triangular form and 

 proportionate size" as in Steneqfiber. "The temporal fossoe", he continues, 

 "appear to have had the same form and proportional capacity. They were 

 separated in the same manner by a long sagittal crest, extending forward upon 

 the frontal bone. The cranium, just back of the forehead, was equally con- 

 stricted. The external auditory passage formed a short, oblique canal, with 

 its orifice directed outward and backward in the same manner. The palatal 

 region, likewise, had the same form and construction, and the infra-orbital for- 

 amen held the same relative position as in Steneqfiber viciacensis. The inci- 

 sors in both jaws are proportionately as long and strong as in the Beaver, 

 and they have the same form." Dr. Leidy adds that they also strongly 

 approach in form those of the Beaver. 



The molar teeth in " Palceocastor" differ very greatly in structure from 

 those of either Castor, Eucastor, or Trogontlieriuin, more resembling, appar- 

 ently, as do also those of Steneqfiber viciacensis, those of some members of 

 the Dasyproctidce. The skull also differs greatly in form from that seen in 

 these genera in consequence of the much greater constriction above of the 

 interorbital region. Hence "Palceocastor", if really belonging to the family 

 Castorida, probably differed very much in general structure from the existing 

 Beavers, and has its nearest ally in the genus Steneqfiber^ of the Miocene 

 epoch of Europe. 



* Zool. et paldont. franc., plates i, viii, anil xlviii. 



t Gervais says, "Les genres Chalicomys, Kaup, et Stcncofihcr, E. Geoff., sont evidemment do la m6me 

 tribu quo les Castors" i Zool. et paloont, franc., p. 20), to winch group they have been generally refeiTed 

 by subsequent writers. Mr. E. R. Alston, however, in his recent memoir " On the Classification of the 

 Order Glircs " (P. Z. S., 1*7(1, pp. 61-1)8), gives both Chalicomys of Kaup and rala'ocastor of Leidy among 

 tlic " more doubtful fossil genera " of the family Castorida, as well as the genera Palaom'js and Chtiodua 

 of Kaun, and the genus Trogontherium of Fischer (not Trogontherijm of Owen, — > Ihobroticus, Pomel). 



