CONTENTS OF A BONE CAVE. 7 



the internal, and they are well separated by the rotular groove, which is here formed 

 by the meeting of planes of different angles. There is little indication of fibular 

 surface on the external side of the astragalus, but the internal side of the inner 

 rotula bears an oblique .facet. This forms an angle of 91 with the rotula proper, /fc?' 

 which angle is directed upwards. Outwards and backwards from the base of this! 

 rotula extends the process already described. The head of the astragalus is short, 

 narrow, and defined by a neck. Its navicular face is subround, slightly convex, and 

 in the transverse plane. The head bears on its external side a facet for a bone, 

 perhaps the cuboid, as in the aguti. On the inferior face the external calcaneal 

 face is concave, and much larger than the internal, which is plane. They are com- 

 pletely separated by a groove which runs out on the postero-interior process 

 already mentioned. 



Of the hinder foot there is preserved a mass which includes the navicular, ecto- 

 and mesocuneiform, and the second and third metatarsals. There is a loose fourth 

 metatarsal, which presents a proximal face for the fifth. The metatarsals are stout, 

 indicating a plantigrade foot. The navicular is not elongate, and has considerable 

 horizontal extent. A part only of the proximal surface is occupied by the concave 

 facet for the astragalus ; the remaining portion extends outwards and backwards, 

 and supports a couple of facets at its anterior border, one superior, the other ante- 

 rior. Corresponding faces on the navicular bone of various rodents are in contact 

 with the entocuneiform distally, and with an additional internal tarsal bone proxi- 

 mally, the internal navicular. These facets, therefore, indicate the existence of a 

 hallux, but perhaps of a rudimental one. The mesocuneiform is smaller than the 

 ectocuneiform, having much less transverse and little less longitudinal diameters. 

 Neither are produced posterior to the proximal ends of their corresponding meta- 

 tarsals. The third and fourth phalanges are about equal in length, and a little 

 exceed the second. Their distal extremities have a prominent inferior median 

 keel. At this articulation there is an ovate sesamoid bone on each side. A single 

 proximal phalange, probably belonging to the A. inundata, is very much depressed 

 at the distal articular extremity indicating a rudimental digit, doubtless the hallux, 

 similar to that found in several existing allied genera. 



The hinder foot of Amblyrliiza then is rather short, furnished with four developed 

 digits and a rudimental hallux, and was probably plantigrade. The lack of tibial 

 crest indicates that the knee was not constantly maintained in a flexed position. 

 The immense trochanter indicates great power of extension of the femur, but whether 

 this extension was effective in running or kicking is uncertain. The absence of 

 tibial crest and the shortness of the foot militate against the supposition that these 

 animals possessed powers of leaping, and their swimming powers would be impaired 

 by the same structural characters. 



Affinities. This genus clearly enters Prof. Brandt's division of the Rodentia 

 which he terms the Hystricomorpha. The evidence is seen primarily in the free 

 fibula and in the development of the angular portion of the mandible on the 

 external side of the incisive alveolus. The small coronoid process and the generic 

 characters add to the weight of the evidence. Mr. E. R. Alston has recently 

 published a very valuable resume of the characters of the subdivisions of the 



