704 MAMMALIA. 



and the enamel is very much plaited, forming a series of 

 transverse ridges enclosing the dentine, and united to one 

 another by cement. Thus on the worn tooth there are 

 numerous successive layers of enamel, dentine, and cement. 

 Extinct forms show transitions between this complex type 

 and the horse's tooth. In a lifetime there may be six molar 

 teeth on each side of each jaw, but of these only one, or 

 portions of two, can find space at a time. The series 

 gradually moves forward as the front parts are worn away 

 and cast out. 



The brain is highly developed. 



The stomach is simple, and there is a large caecum. 



There are two superior venae cavae entering the right 

 auricle. 



The testes remain abdominal in position. 



There are two pectoral mammae; the uterus is bicornuate; 

 the placenta is non-deciduate and zonary. 



106 

 Elephas, , now represented by the Indian Elephant (. indicus), 



with parallel folds of enamel on the molars and ears of moderate size, 

 and the African Elephant (E. africamis], with lozenge -shaped folds of 

 enamel and very large ears. 



The mammoth (E. primigenius] belonged to the Pleistocene period, 

 and had a wide geographical range, occurring for instance in Britain. 



The genus Mastodon is represented by fossil remains in Miocene, 

 Pliocene, and even in Pleistocene strata, in Europe, India, and America. 

 The molar teeth show transitions between those of elephants and those 

 of other Ungulates. 



In Dinotherium , found in Miocene and Pliocene strata in Europe and 

 Asia, the lower jaw bore an enormous pair of tusks projecting vertically 

 downwards, and all the back teeth seem to have been in use at the 

 same time. 



SEVERAL EXTINCT SUB-ORDERS. 



Although we cannot describe the following remarkable types, it is 

 important to notice their existence, for they serve to impress us with the 

 original connectedness of what are now separate orders. 



The huge Amblypoda, found in Eocene formations in W. America, 

 had three pairs of remarkable protuberances on the top of the skull, no 

 'upper incisors, large upper canines, especially in the males, and six back 

 teeth. Example Uintatherium ; the genus Coryphodon may also be 

 related. 



Cope includes a number of generalised Eocene Ungulates under the 

 title Condylarthra. Some seem ancestral to the Perissodactyla and 

 Artiodactyla ; some suggest a union of ancestral Ungulates and ancestral 

 Carnivores. The genus Periptychus may be regarded as an ancestral 



