422 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 



Wot Indies. These forms are thus far known only from the detached teeth 

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

 ured 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 laminae. The incisors are also much smaller and 

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

 of the molars of Amblyrhiza have four dentinal columns and others 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 Archceomys, a European form commonly referred to the 

 Chinch illidce* Hence the question naturally arises whether the Chinchillidce 

 have yet been found outside of South America. The discovery of a single 

 mandibular ramus, or the facial portion of the skull for each of these genera, 

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

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

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

 South America. 



Although Castoroides has 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; but in its dentition the type is wholly different, as is also the confor- 

 mation of the pterygoid processes and fossae. ...... In the Hydrochcerus, 



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



" Mr. Alston (Proc. Z06I. Soc.Loud , 18/6, p. 88) refers Archaomys to the family Theiidomyidce, with 

 the other forms of which, however, it does not seem to me to be very closely related. 



t From the absence of all reference by Dr. Wyman to the much closer resemblance of the teeth of 

 Castoroides to those of the Chinchillas, he was evidently not at that time acquainted with the osteology 

 of that group. 



