429 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
West 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 Castorozdes, having, in fact, 
the same structure, differing mainly in being somewhat smaller, and in pos- 
sessing a greater number of laminz. 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 
Chinchillide, or whether they are not more closely allied to Castoroides. The 
same may be said of Archeomys, a European form commonly referred to the 
Chinchillide.* Hence the question naturally arises whether the Chinchillide 
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 
Hydrocherus. 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 fosse.- ..... In the Hydrocherus, 
the principal analogies are found in the compound nature of the molar teeth.t 
* Mr. Alston (Proc. Zod]. Soc. Lond, 1876, p. 88) refers drchwomys to the family Theridomyide, with 
the other forms of which, however, it does not seem to me to be very closely related. 
+ 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. 
