386 PATAGONIAN EXPEDITIONS I PALAEONTOLOGY. 



Recent. Santa Cruz. 



Lagidium. Scotaeumys. 



Perimys. 

 Sphodromys. 



IV. DASYPROCTID/E. 



Dasyprocta. Not found. 



Agouti. 



V. DlNOMYID/E. 



Dinomys. Not found. 



VI. CAVIID/E. 



Cavia. Eocardia. 



Kerodon. Schistomys. 



Dolichotis. Phanomys. 

 Hydrochoerus. 



29 20 



The Santa Cruz rodents are, of course, somewhat more primitive in 

 structure than their recent successors, but the difference is hardly so great 

 as might have been expected. In most instances, the teeth have a some- 

 what simpler pattern, but in others the modern pattern is fully attained 

 and in a few cases surpassed. The skull structure is very nearly the same 

 in the two faunas, with very minor differences. For example, the palate 

 is rarely so extremely triangular as in many of the modern genera ; the 

 coronoid process of the mandible is less reduced in size and the masse- 

 teric crest is more normal in development. Limbs and feet are usually, 

 but not always, relatively smaller and the ulna and fibula have under- 

 gone little reduction. The number of digits is known in only a few of 

 the Santa Cruz genera and in most of these the formula is V-V, but in 

 the cavies the number IV-III is already characteristic. 



From a phylogenetic point of view the Santa Cruz rodents are of much 

 interest. In several of the genera, there is every reason to believe, we 

 have the direct ancestors of existing forms : for example, l^iscaccia was prob- 

 ably derived from Prolagostomus, Dolichotis from Schistomys, Rrethizon 

 and Coendou from Steiromys. Several other genera may also be regarded as 

 probable ancestors of recent types, but are still too incompletely known 

 for any definite determination of their relationships. Thus, Scotceumys 

 is the probable forerunner of Chinchilla and Lagidium, and the modern 



