12 
PATAGONIAN EXPEDITIONS : PALAEONTOLOGY. 
takes part in forming the outer portion of the glenoid cavity and the 
parietal enters into the postorbital process. Neither of these characters 
is exhibited by the Typotheria. Again, in the Hyracoidea, the first upper 
incisor of the permanent series is a persistently growing, downwardly 
curved tusk, triangular in section, while in the Typotheria this tooth, 
although growing persistently in some forms, is always antero-pos- 
teriorly compressed, transversely expanded and functional as a cropping 
tooth. The molars in Procavia are lopho-selenodont and 
either brachyodont or short hypsodont, while in the Typo- 
theria they are extremely hypsodont, developing roots only 
in the deciduous series. In crown-pattern, they bear less re- 
semblance to the teeth of the Hyracoidea than do the molars 
of the early horses and rhinoceroses, which differ from Pro- 
cavia as fundamentally in foot-structure as do the Typotheria. 
The so-called Hyracoidea from the Fayum of Egypt ( SctgJia - 
therium, Megalohyrax ) are as yet known only from fragments 
of the skull and dentition. It would naturally be supposed 
that they should bear a closer resemblance to the Miocene 
Typotheria than to the recent hyraces, if the two groups 
are related. So far as the available material permits com- 
parison, this is not found to be the case, the Egyptian forms 
showing no closer approximation to the Typotheria than do 
the modern hyraces. 
Mr. Walter Granger has called the writer’s attention to the 
apparently constant presence in the Hyracoidea, as well as 
in these so-called hyracoids from the Fayum, of a superior branch of the 
alveolar canal, which perforates the base of the coronoid process of the 
mandible behind the last molar, as in Lepus and the Santa Cruz dipro- 
todont marsupial Abderites . Although thus shown not to be strictly 
confined to the Hyracoidea, this perforation is conspicuously absent in the 
Typotheria and may be interpreted as a further indication of their lack 
of Hyracoidean affinities. 
Various pre-Santa Cruz genera (. Archceohyrax , Argyrohyrax ) have 
been referred to the Hyracoidea. The foot-structure of these is unknown, 
but the skull and dentition, in the writer’s opinion, are not hyracoidean 
in character. Too little is known of these forms to warrant a discussion 
of their relationship to the Santa Cruz Typotheria, but from the available 
Fig. 7. 
m 
Dcndrohy- 
rax arbor eus, 
left pes, dor- 
sum, x | (No. 
365, Prince- 
ton University 
osteological 
collection). 
