8 CONTENTS OF A BONE CAVE. 



Rodentia, including the genera. He divides the Hystricomorplia into numerous 

 families, some of which at least appear to the writer to rest on rather slender bases. 

 In the comparison with Amblyrhiza, the Ilystricidce and Dasyproctidce may be 

 dismissed from the fact that their molars are not divided transversely by laminae 

 of enamel. The comparison is with the CJi inch ill idee and Caviida:. The molar 

 dentition is that of the former family, and the absence of a masseteric ridge sepa- 

 rates it from the genera arranged by Mr. Alston under the Caciidce, although I 

 cannot perceive that such a character should define a family group. The incisors 

 of both these groups are called by Mr. Alston "short." I have shown those of 

 AmblyrJiiza to be very long, as in the Dasyproctidce ; nevertheless, their transverse 

 section and sculpture are much as in the genus Lag-idium. The affinities of this 

 form are then near to types now existing on the South American Continent, but it 

 presents characters which show that it cannot be referred to any existing genus. 

 The extinct Arcliceomys, Laiz., Par., of the French Miocene, resembles it in the 

 constitution of its molar teeth, but whether in other respects or not I cannot 

 ascertain. There is one less dentinal column in the posterior superior molar of 

 ArchcKomys, so that the formula for those teeth reads -| in AmblyrJiiza and -*- in 

 Archceomys. 



The only other extinct form with which it is necessary to make comparison is 

 the one called by D'Orbigny, Meyamys patagoniensis. This species rests on a tibia 

 found in Patagonia, and it is described and figured in the Voyage en Amerique 

 Meridionale, Vol. III. PI. VIII. figs. 4-8. Although the specimen was injured, 

 enough remains to show that it belongs to a different species and genus from any 

 of those herein enumerated. The differences are to be seen in the distal extremity. 

 They are, first, the absence of the divergent internal malleolar process, which is so 

 striking in AmblyrJiiza ; second, the large size of the internal cotylus of the tibia, 

 indicating that the internal rotula of the astragalus is as large as the external one, 

 instead of being much smaller. The deep fissure which separates the articular 

 faces of the head of the tibia is similar to that seen in AmUyrhista. 



My friend, J. A. Allen, the distinguished zoologist, remarks in his article on the 

 Castormdidoe :' "To the same group (i. e., the Castoroididce) are, however, probably 

 referable the genera AmblyrJiiza and Loxomylus, described from the bone caverns 

 of Anguilla Island, West Indies." . . . "The molars as described and figured 

 by Professor Cope greatly resemble those of Cas/oroidcs, having, in fact, the same 

 structure, differing mainly in being somewhat smaller and in possessing a greater 

 number of laminaB. ... 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 

 CJiincJiillidce or whether they are not most closely allied to Gas/oroides." The 

 description of the lower jaw and other parts of the skull found in the preceding 

 pages show conclusively that AmblyrJiiza has little affinity with the Castoroididce, 

 although, as Mr. Allen remarks, there is close resemblance between the molar teeth 

 of the two forms. Castoroides differs from AmblyrJiiza in its generally Sciuromor- 



1 Final Report of the U. S. Geological Survey of Territories, vol. xi. p. 421. 



