MAMMALIA. 345 



the lines the teeth in the lower jaw. The milk dentition is 

 expressed by a similar formula with d (deciduous) prefixed to 

 the letter expressing the nature of the tooth. 



The following terms are of frequent use as characterising 

 certain forms of the grinding surfaces of teeth, and it will be 

 well to define them at once. 



Bunodont is a term applied to teeth with broad crowns 

 raised into rounded tubercles, e.g. the grinding teeth of Pigs 

 and Hippopotami ; 



Bilophodont to teeth marked by a simple pair of trans- 

 verse ridges, with or without a third ridge running along the 

 outer border of the tooth at right angles to the other two, 

 e.g. the grinding teeth of Lopkiodon, Kangaroo, Manatee, 

 Tapir, Dinotheriu7n ; 



Selenodont to teeth marked by crescentic ridges running 

 from the anterior towards the posterior end of the tooth, e.g. 

 the grinding teeth of the Ox and Sheep. 



Teeth whose crowns are low so that their whole structure 

 is visible from the grinding surface are called brachydont, while 

 those with higher crowns, in which the bases of the infoldings 

 of enamel are invisible from the grinding surface are said to 

 be hypsodont. Bunodont teeth are brachydont, the teeth of the 

 Horse and Ox are hypsodont. 



Passing now to the appendicular skeleton the shoulder 

 girdle differs markedly from that of Sauropsids in the fact that 

 the coracoid, except in the Ornithodelphia, is greatly reduced, 

 generally forming only a small process on the scapula. In the 

 pelvis the pubes meet in a ventral symphysis, except in some 

 Insectivora and Chiroptera. In many mammals a fourth pelvic 

 element, the acetabular bone, is distinguishable. The ankle 

 joint is cruro-tarsal, or situated between the proximal tarsal 

 bones and the tibia and fibula. Carpalia 4 and 5 are united 

 forming the unciform ; and the ulnar sesamoid bone or pisiform 

 is generally well developed. In the proximal row of tarsal ele- 

 ments there are only two bones, the calcaneum and astragalus. 



