114 



RODENTIA 



ORDER VI 



Subfamily 6. Caviinae. 

 Cheek teeth composed of two, seldom more, angulated prisms. 



Cephalomys Ameghino. Pyrotherium beds of Patagonia. 



Eocardia, Procardia, Schistomys and Phanomys Ameghino. Santa Cruz. 

 Ancestors of Cavia. 



Cavia Pallas. Dolichotis Desmarest. Kerodon Cuv. Recent and Pleistocene 

 of South America. Ameghino has established several new genera, Cardiodon, 

 Anchimys, etc., upon Pliocene remains foreshadowing Cavia. 



Hydrochoerus Brisson (Fig. 128, p. 102). Capybara. The largest Recent 

 rodent. Fossil in the Pleistocene of South and North America. 



m 



Fia. 146. 



Nesokerodon quercyi 

 Sehlosser. Oligocene. 

 Phosphorites of Quercy. 

 a, Mandible with P^ (young 

 specimen) ; 6, upper D^ ; 

 c, lower Oj. sj-^. 



Subfamily 7. Issiodoromyinae. 



This subfamily, restricted to the European Oligocene, 

 may be considered on account of its dental formula and 

 structure as transitional between the Theridomyinae and 

 Caviinae. D with complicated structure and functioning, 

 P and M rooted. 



Nesokerodon Sehlosser (Fig. 146). Teeth still dis- 

 playing outer and inner enamel folds and fossettes. 

 Phosphorites of Quercy. 



Issiodoromys Croizet. Oligocene ; Dept. Allier. 



Suborder B. DUPLICIDENTATA (Lagomorpha). 



■f- /, entirely covered with enamel. 



3-2. 



directly behind F^. P #. M 



3-2. 



Second incisor — P — is small and placed 

 Cheek teeth hypsodont, without roots and 



made up of two, less frequently of three, angidar prisms. The distance between the 

 two upper dental rows is greater than between the lower. The socket for the condyle 

 of the lower jaw is shallow. Carpals free. Fibula articulating with calcaneum. 



This group, commonly called Lagomorpha, is distinguished from remaining 

 rodents in having two upper incisors. These teeth are entirely covered with 

 enamel, although the layers on the back and sides are considerably thinner 

 than on the front surface. The larger of the two upper incisors has a longi- 

 tudinal depression. The cheek teeth are without roots, open below, and the 

 upper ones rather broader than long in cross-section. They are composed of 

 prisms which appear to be compressed in an antero-posterior line and con- 

 nected by cement. The inflections between the prisms of the lower teeth 

 are all of the same depth, but the outer ones of the upper teeth are shallower 

 than those on the inner surface. The first upper P and the last lower M 

 have sometimes only a single prism, but the last lower M has, at times, three 

 prisms. The first lower P and the first two upper P often are structurally 

 ditferent from the M. Sometimes enamel fossettes and ridges can be seen 

 on the crown, as evidence of their bunodont and lophodont origin. Low, 

 rooted B precede the P, and are shed as soon as the last M begins to function. 

 In the milk dentition there are in the upper jaw three deciduous /. 



