I 



lii I 



liil^'l! 



fc: 



422 



MONOGKAPHS OP NORTH AMERICAN RODENTIA. 



West Indies. Tliese forms are thus far known only from the detached teeth 

 and fragments of the hones of the limbs. The molars, as described and fig- 

 uied by Professor Cope, greatly resemble those of Castoroides, having, in fact, 

 the same structure, differing mainly in being somewhat smaller, and in pos- 

 sessing a greater number of laminsB. The incisors are also much smaller and 

 narrower, and much loss strongly grooved. Professor Cope states that some 

 of the molars of Amblyrhiza have four dentinal columns and otiiers five, 

 while those of Loxomylus, including both upper and lower, have only three 

 each. The characters of Amblyrhiza, as Professor Cope recognizes, ally it to 

 the Chinchillas, while he says of Loxomylus that the obliquity of "the hori- 

 zontal grinding surface . . . alone seems to distinguish it from Lagidium 

 and Chinchilla". As the lower jaw and skull are thus far unknown in these 

 genera, it is impossible to say whether their affinities are strictly with the 

 Chinchillidee, or whether they are not more closely allied to Castoroides. The 

 same may be said of Archeeomys, a European form commonly referred to the 

 Chinchillidee.* Hence the question naturally arises whether the ChinchillidcB 

 ha\ it been found outside of South Amerca. The discovery of a single 

 manu.uular ramus, or the facial portion of the skull f" each of these genera, 

 would at once decide the question cf their affinities, which cannot well be 

 settled without the evidence such parts would affisrd. In either case, these 

 genera furnish a type of dentition unknown in the present fauna, except in 

 Soutii America. 



Although Castoroides ha.s generally been supposed to have the relation- 

 ship to Castor its name implies, and in systematic works has been always 

 associated with the Beavers, Dr. Wyman, in his monographic account of the 

 Clyde skull, points out the great differences that exist between the two types. 

 He says the cranium "presents analogies to the genera Castor, Fiber, and 

 Hydrochcerus. Osteologically considered, the cranium bears a stronger resem- 

 blance in its shape to that of the Castors than to that of either of the other 

 genera; Jut in iu dentition the type is toholly different, as is also the confor- 

 mation of the jitor >id processes and fossiB In -thT Hydrochcerus, 



the principal analogies are found in the compound nature of tlie molar tceth.f 



• Mr. Alston (Proc. Zool. Soc.Lond , lh7C, p. 88) refers .irchaomyf to the fnmily Thtiiiiomyidai, with 

 the otbor fo" ■ if wliicli, liowevr. it does iiot setm to me to bo very cloi.ely related. 



t l''roni -lie ubiieiice of nil reference by 'i''. Wyman to the much closer reBewulance of the teeth of 

 tailoroUis to those of the Chiuchillas, he was cMdently not at that time acquainted with the osteology 

 of that group. 



