388 PATAGONIAN EXPEDITIONS I PALEONTOLOGY. 



least, of all the South American families of the suborder. It will, I think, 

 be eventually possible to prove that all the varied molar patterns of the 

 existing genera have been derived, now by simplification, again by increased 

 complication, from a pattern which is nearly exemplified by the genera 

 Luanthus and Cephalomys of the Patagonian beds. 



This conclusion agrees in a general way with the results of Winge ('87, 

 128) who assumes that the grinding teeth were early complicated by the 

 formation of four more or less complete, transverse folds, though I should 

 regard three as the more probable number of such folds. 



Tullberg, on the other hand, does not accept this conclusion : "Mehrere 

 Griinde lassen es meines Erachtens einfacher erscheinen, fur die Back- 

 zahne der Hystricomarphen eine allmahliche und bei den verschiedenen 

 Formen selbstandig je nach Bedarf eintretende Komplizierung auzuneh- 

 men. Es 1st natiirlicherweise nicht unmoglich, dass Falten bei dieser 

 oder jener Form, welche deren mehrere an jedem Zahn besass, reduziert 

 worden sind. Daraus diirfte indes keineswegs zu folgern sein, dass die 

 urspriinglichen Hystricomorphi ebenso zahlreiche Falten gehabt hatten, 

 wie Z. B. die Erethizontiden. Falls man dieses behauptete, miisste man 

 namlich annehmen, dass die Falten bei Cavia und Dolichotis reduziert, 

 aber bei Hydrocheerus an Zahl vermehrt, bei Petromys und Aulacodus 

 und dieses ebenfalls bei alien Eckinomyiden, Myopotamus ausgenom- 

 men, reduziert worden waren " ('99, 365). 



So far as the cavies are concerned, the assumption which Tullberg 

 regards as so improbable is demonstrably true. It will be shown in a sub- 

 sequent chapter that in Schistomys, which is almost certainly the direct 

 ancestor of Dolichotis, the unworn teeth have three transverse enamel 

 folds, in addition to the principal valley between the prisms. These folds 

 are soon converted by abrasion into enamel lakes and in the fully adult 

 animal they are entirely worn away, leaving the teeth exactly like those of 

 Dolichotis. In Luanthus, of the pre-Santacruzian, or Patagonian beds, 

 the pattern of the teeth is the same as in Schistomys, with the important 

 exception that the enamel folds are deeper and the teeth are rooted. 

 Here, then, we have clear evidence that in Dolichotis, at least, the exist- 

 ing type of tooth has been acquired by simplification. 



The Santa Cruz rodent fauna throws no light whatever upon the prob- 

 lem concerning the relationships of the Hystricomorpha to the other rodent 

 suborders. For this purpose the fauna is of much too late a period to be 



