Sinclair: typotheria of the santa cruz beds. ii 
Hystricidse and Ccelogenys , and the general fusion of scaphoid and lunar in 
all except the Bathyergidae, Ctenodactilidse and the Lagomorpha (Weber, 
1904, p. 476). The centrale is always wanting in the carpus of the Typo- 
theria and the fusion of the scaphoid and lunar does not occur. 
8. Presence of a tibial sesamoid in all simplicidentate rodents. This is 
not present in the tarsus of the Typotheria. 
The Typotheria resemble rodents in the elongation of the anterior 
portion of the skull, with reduction in the incisor-canine-premolar series, 
in the enlargement and often permanent growth of the median incisors 
(not homologous with enlarged incisors in rodents, see under 1, p. 10), in 
the development of a mastoid dilatation, which maybe filled with cancellae 
(Interatheriidae), as in many rodents, and connected with the auditory bulla, 
in the shape of the proximal articular surfaces between radius and ulna 
(see Pis. II, fig. 6; IV, fig. 16), in the broad, anteriorly directed transverse 
processes of the lumbar vertebrae and in several other characters of minor 
importance. 
In view of the striking differences in structure indicated in the preceding 
paragraphs, it seems impossible to interpret these resemblances otherwise 
than as due to convergence. 
IV. With the Hyracoidea. 
Various writers have suggested a more or less intimate relationship 
between the Typotheria and the Hyracoidea, which the complete material 
now available has failed to substantiate. In the Hyracoidea the carpus is 
arranged on the linear plan with separate os centrale, while in the tarsus 
the astragalus differs from that of any other mammal in possessing a large 
step-like articulation for the internal tibial malleolus (text fig. 7). On the 
contrary, in the Typotheria the carpus is strongly interlocking, without 
centrale, and the internal tibial malleolus is applied to the lateral surface 
of the trochlea, with no trace of the supporting shelf characteristic of the 
Hyracoidea. The flat head of the astragalus and the articulation of the 
fibula with the latter element instead of with the calcaneum also serve to 
separate the Hyracoidea from the Typotheria. These differences in foot- 
structure are more than sufficient to offset similarities in the skull, which 
are confined to a few points, such as the cancellous dilatation of the mastoid, 
the shape of the posterior border of the palate and the increase in depth 
posteriorly of the mandible. In all living species of Hyracoidea the malar 
