1256 



CLASS MAMMALIA. 



Cub 



geals repeat the characters of the homologous bones of the manus, 

 although they deviate in a lesser degree from a common type. 

 In the Cetacea and Sirenia the pes is entirely absent ; although the 

 proximal portions of the pelvic limb may be detected in an aborted 

 form in many genera. 



We may now take a brief glance at the general distribution in 

 time of the Mammalia, in the course of which we shall have to fore- 

 stall some of the information given in the sequel. 



If the theory of evolution be the true explanation of the order 

 of nature we should expect to find that the earliest representatives 



of the class would be small forms, 

 more or less closely related to one 

 another, not numerous in generic types, 

 and allied to those orders which now 

 comprise the most generalised repre- 

 sentatives of the class. Our knowledge 

 of these earlier types is indeed exceed- 

 ingly imperfect, but so far as it goes it 

 appears to accord fairly well with the 

 foregoing conditions ; all the earlier 

 forms being of small size, and appar- 

 ently more or less closely allied to the 

 existing Marsupials, and probably also 

 to the Monotremes and perhaps the 

 Insectivores. The earliest evidence of 

 the occurrence of Mammals yet known 

 is in the upper portion of the Triassic 

 system, which forms the base of the 

 great Mesozoic period. Of the two 

 known genera from these deposits the 

 larger type, termed Dromatherium, ex- 

 hibits some curious approximations in the structure of its teeth to 

 Reptiles and Amphibians, and it is possible that we may have in 

 this form an ancestral type of the Prototheria. In the Lower 

 Jurassic, immediately above the Lias, remains of small Mammals 

 become more common, and in the Upper Jurassic they are locally 

 abundant, although they comprise comparatively few generic types. 

 These Jurassic Mammals are readily divisible into two groups, distin- 

 guished by the characters of their teeth. Of the first group an ex- 

 ample of the lower jaw is shown in fig. 1141, p. 1274, where it will 

 be seen that the teeth are very numerous, the hinder ones having 

 several sharp cusps on the crowns. Although there has been much 

 discussion as to the affinities of these small Mammals, it is probable 

 that they are really Marsupials of the suborder Polyprotodontia, 

 with a relationship to the Australian Anteater (Afyrmecoh'us), which, 





Fig. 1 1 29. — Dorsal aspect of the 

 right pes of Man. Reduced. Ca, 

 Calcaneum (fibulare + ? pisiform) ; 

 As, Astragalus (intermedium) ; c, 

 Navicular (centrale) ; Cu, 1-3, Ento-, 

 meso-_, and ectocuneiform (1st, 2d, 3d 

 tarsalia) ; Cub, Cuboid (4th, 5th tar- 

 salia) ; i-v, Metatarsals. (After Wie- 

 dersheim.) 



