742 MAMMALIA. 



The dentition is unique. The two upper incisors or 

 tusks are mainly composed of solid ivory ; the enamel is 

 restricted to the apex, and soon wears off. As the tusks 

 grow, their roots sink through the premaxillae into the 

 maxillae. There are no canines nor premolars. The 

 molars are very large, 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 vense 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. 



Elt'phas, - -, now represented by the Indian Elephant (E. inth'cns}, 



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

 and the African Elephant (E. africanus), with lozenge-shaped folds of 

 enamel, and very large ears. 



The mammoth (E. pritingenins} 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 Dinotheriiun, 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, in Eocene formations in America and Europe, 



