ORDER MONOTREMATA. 



1269 



has also been described from the same deposits, and is regarded as 

 indicating a distinct family. 



Family Polymastodontid^e. — The type genus Poly mastodon 

 occurs in the Lowest Eocene of North America, and has one pre- 

 molar and two true molars. The premolar is tubercular, and the 

 upper true molars have three longitudinal ridges, and are elongated 

 antero-posteriorly. 



Family Tritylodontid^e. — The genus Tritylodon was first de- 

 scribed from a nearly entire cranium found in the Karoo system of 

 South Africa, in a horizon which is probably of Lower Mesozoic 

 age ; but a tooth (fig. 1 136) previously 

 obtained from the Upper Trias of 

 Germany, and described under the 

 preoccupied name of Triglyphus, 

 proves to belong to the same genus. 

 The upper dental formula is /. 2, C. o, 

 P?n. 2, M. 4. The innermost upper 

 incisor is large and scalpriform, while 

 the outer one is very minute. The 

 upper true molars (fig. 1136) carry 

 three longitudinal ridges, and have the 

 longer diameter of the crown directed 

 transversely. The anterior portion of 

 the cranium is remarkable for its great 

 width and bluntness. The African 

 species indicates an animal about the 

 size of a Rabbit. From the same de- 

 posits at the Cape has been obtained 

 a slab showing the impression of a 

 pectoral limb apparently referable to a 



small Mammal, which has been described under the name of Ther- 

 iodesmus. If, as is probably the case, this specimen be really 

 Mammalian, there is, however, no reason why it should not belong to 

 Tritylodon. According to Professor Bardeleben this limb has two 

 centralia in the carpus (as in some Insectivora), and a distinct pre- 

 hallux ; the alleged fusion of the scaphoid with the lunar being in- 

 correct. To the present family may likewise be referred the very 

 imperfectly known Stereognathus, of the Lower Jurassic of Stones- 

 field ; the upper cheek-teeth of which closely resemble those of Tri- 

 tylodon. Finally, Chirox, from the Upper Jurassic of North America, 

 shows certain characters connecting the present with the next family, 

 in which it is provisionally placed by Professor Osborn. 



Family Bolodontid^e. — Nearly allied to the preceding family, 

 but with the upper true molars antero-posteriorly instead of trans- 

 versely elongated, and with only two longitudinal ridges, is the 



Fig. 1136. — An upper true molar of 

 Tritylodon Fraasi; from the Upper 

 Trias of Strasbourg. The two central 

 figures are of the natural size ; the 

 others enlarged three times, o, Crown 

 surface ; u, Basal surface ; v, h, The 

 two lateral surfaces ; i, «, Anterior and 

 posterior surfaces. The position of o 

 is at right angles to that of the molars 

 in the next figure. 



