TILLODONTIA. 607 



bulk of them are found in the Eocene, though they extend 

 through the Miocene into the PHocene. 



Their general characters may be described as follows. They were small 

 animals, about the size of Hyrax. The brain cavity is small and the brain 

 was smooth ; the orbit is not closed though the frontal has a well 

 marked postorbital process. The skull on the whole recalls that of Hyrax, 

 and in certain features that of rodents and of the Toxodontians. The 

 dentition is in the oldest forms complete and almost in a closed series. It 

 is in many respects like that of the Toxodontia. The grinders are hypso- 

 dont and rodent Mke. In the older forms the teeth seem to have been 

 rooted, but in the more recent they were rootless. The scapula has a 

 coracoid process and its spine has an acromion and a peculiar backwardly 

 directed process as in elephants and some rodents, and there is a clavicle. 

 The humerus has an entepicondylar foramen. The ulna and radius are 

 separate and capable of rotation. The fibula articulates with the cal- 

 caneum and is separate from the tibia. The femur has a third trochanter. 

 The carpus has a centrale in the older forms, and the arrangement of the 

 two rows of bones is successional in the other, alternating in the more recent 

 forms. The manus possesses five digits, and the distal phalanges are 

 either broadened and hoof-like or split, like that of digit No. 2 of the foot 

 of Hyrax. The pollex appears in some cases at any rate to have been 

 opposable. In the foot the distal phalanges are broadened and the hallux 

 was opposable. The sacrimi was composed of seven vertebrae as in some 

 edentates. Protypotherium Am., i i c \ p ^ m ^ in a nearly closed series ; 

 carpalia serial, with centrale, the pes has 5 digits, the fibula articulate 

 with the calcaneum. Eocene and Miocene. I cochilus Am., 'Eocene. Typo- 

 iherium Bravard, i i c § p ^ m i, with wide diastema, grinders rootless ; 

 carpaUa alternating, without centrale ; fibula articulating with astragalus ; 

 pes tetradactyle ; attains the size of a pig ; Miocene, Pliocene. 



Order 12. TILLODONTIA.* 



This is a group of extinct forms from the Lower and Middle Eocene of 

 N. America and according to Ameghino from the Cretaceous of Patagonia, 

 and there are fragmentary remains (Platychaerops Charlesw.) from the 

 Eocene of England. The group is not well known but the characters, 

 so far as ascertained, are as follows. Plantigrade, pentadactyle, with 

 clawed digits ; and rodent-like incisors growing from continuous or long- 

 persistent pulps, and with enamel only on their anterior face ; the u. 

 grinders are tritubercular, the lower tubercular-sectorial. The premaxillae 

 are large ; there is no postorbital process on the frontal ; the humerus has 

 an entepicondylar foramen, and the femur a third trochanter. The skeleton 

 presents resemblances to the Camivora. The dentition when complete is 

 * I c ^ p 3 m §, but the first and third incisors are smaller than the second 

 large rodent-like pair and may be absent. The canines also tend to be 

 reduced. The brain was small and weakly furrowed. The Tillodontia 

 were large and medium-sized land animals with likenesses to the Carnivora 

 and Rodentia and are often associated with the latter in classification, 



* Marsh, Tillotherium, Amer. Journ. Sci. (3), 9, 1875, p. 221, and 11, 

 1876, p. 249. Cope, Vertebrata of the Tertiary Form, of the West, 1S77. 

 Wortman, Ganodonta, Bull. Amer. Mus. N. Hist., 9, 1897, p. 59. 



