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 I2 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 3-3 (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. 
