EXTINCT UNGULATE MAMMAL EEOM PATAGONIA. 
179 
dactyle all the premolars, even the last, are structurely reduced, as compared with the 
true molars, so that a more or less obvious break in the continuity of the appearance of 
the teeth is seen between the last premolar and the first true molar. On the other 
hand, in many Perissodactyles, including all the existing representatives of the group, 
several of the posterior premolars are very close repetitions of the true molars in 
structure and even size ; though it must be noted that this was not the case with the 
earliest known forms of the group, the Coryphodons, Lophiodons, and Hyracotheriums, 
and to a less extent the Palaeotheriums, which so far approximate to the Artiodactyles, 
or perhaps rather to a more generalized Ungulate type, of which no representatives have 
as yet been discovered. 
The similarity of the structure of the premolars and true molars of Homalodontotherium, 
removes it from the vicinity of all known Artiodactyles. The special characters of the 
crowns of the molars of both jaws are yet more decisive ; they fall into neither the 
Bunodont nor the Selenodont division of that group, and, except remot.ely to the somewhat 
aberrant Anoplotherium , present no resemblance to any known type of Artiodactyle. 
On the other hand, as will be shown, they approach the Perissodactyle genus Rhino- 
ceros more closely than to any other known mammal. In order to understand the 
nature and taxonomic value of this resemblance, a few preliminary remarks may be 
necessary. 
As regards the upper molars, the essential character of the crown of Perissodactyle 
Ungulates, as is perhaps best exemplified in Lophiodon , is the presence of four principal 
cusps, arranged in pairs anterior (see fig. 4, aic, aec) and posterior {pic and 
p e c ), connected more or less by transverse ridges, also anterior and posterior {a r and 
p r). The outer cusps are joined together by an antero-posterior ridge, constituting 
the external wall or lamina {l), but there is no corresponding inner wall connecting the 
inner cusps. Between the two transverse ridges is a median sinus (m s ), bounded exter- 
nally by the outer wall, but open on the inner side. Behind the posterior transverse 
ridge is a smaller posterior sinus (p s ), opening on the posterior surface of the tooth, 
or only enclosed by the cingulum, and in front of the anterior transverse ridge is a 
similar but less important anterior sinus {a s). 
Furthermore, the transverse ridges are usually placed obliquely, their outer ends 
inclining forwards and running quite to the front edge of their respective external 
cusps, so that their posterior surface is more or less concave. 
From this type there are three important deviations : — I. That in which the outer 
wall is undeveloped, and the transverse ridges become the prominent features of the 
crown of the tooth, as in the Tapirs. II. That in which the free edge of the outer wall 
acquires a strongly zigzag or bicrescentic character, being deviated inwards opposite 
each of the principal outer cusps, and outwards at the anterior and posterior angles of 
the tooth and in the middle between the cusps, as in Palceotherium , the Horses, and 
apparently (judging from Bravard’s figure of the worn molars*) in Macranchenia. 
* Published in Burmeister’s ‘ Anales del Museo Publico de Tuenos Aires/ vol. i. pi. 1. 
2 A 2 
