88 THE DESEADO FORMATION OF PATAGONIA 



mesostylid of the Fayum hyracoids. Between the pillar 

 and the median horn, I find a narrow vertical ridge, which 

 I have termed the septum; and which tends to unite with 

 the pillar inclosing a small bay, usually seen in worn teeth 

 as a pit. The bay between the septum and the median 

 horn is designated bay 2, and this quite generally appears 

 in a worn tooth as a pit (2). The bay between the septum 

 and pillar is designated bay 3, and is usually seen as a tiny 

 pit, which however does not extend as deep into the crown 

 as the other pits and is usually lost when the tooth is about 

 half worn off. The bay between the pillar and the posterior 

 horn is numbered 4, and is usually open, though in a worn 

 tooth it also may appear as a pit. The effect of wear is 

 shown by comparing B and C in fig. 52, the latter being 

 the same tooth sectioned a little below the middle. I find 

 in studying a lower molar of Coresodon that bay 3 becomes 

 a pit after some 6 mm. are worn off, while bays 2 and 4 

 remain open until some 10 mm. are worn off when they 

 also become pits. Pit 3 will disappear when 12 mm. are 

 worn off, but pits 2 and 4 run to the base of the crown. 



The various genera of the Toxodontia in the Deseado 

 I would divide into four families as follows : 



Rhynchippidae : molars brachydont, secondary cristae 

 lacking or little developed, none of the incisors caniniform, 

 limbs slender, feet digitigrade, digits 3-3. 



Leontinidae: molars brachydont, secondary cristae 

 lacking or little developed, upper inc. 2 and lower inc. 3 

 developed into caniniform teeth, limbs heavy, feet digiti- 

 grade, digits 33 (according to Gaudry). 



Isotemnidae : molars brachydont, secondary cristae 

 more or less developed, crowns contracted at the top, con- 

 gulum more or less developed into a platform, skeleton 

 unknown. 



Nesodontidae : molars hypsodont, secondary cristae 

 highly developed, upper inc. 2 and lower inc. 3 caniniform, 

 limbs heavy, feet digitigrade, digits 3-3. 



