54 



BRITISH MAMMALS 



seven pairs, and in one species of tenrec twelve pairs. Their 

 teeth are remarkable in several ways ; frequently presenting the 

 typical and Eutherian number of forty-four.^ The molars in the 

 generalised types are of simple tritubercular construction ; in the 

 others they are many-cusped. The canines and incisor teeth are 

 normally present in both jaws ; and in regard to the incisors, 

 several species of Shrews exhibit an apparent affinity to the 

 Marsupials, and perhaps to the earlier Mammals, in that they have 

 four pairs in the upper jaw instead of the normal three.^ There 

 is a third trochanter to the thigh bone, and an entepicondylar 

 perforation of the arm bone (humerus, see p. 149). Both these 

 are archaic features. The canines in some species are double- 

 rooted — a peculiar and perhaps a primitive feature met with in 

 that curious animal the flying cobego of Eastern Asia, which 

 has affinities to the Bats, the Lemurs, and the Insectivores. 



In their origin the Insectivores are with difficulty dis- 

 tinguished from the early types that gave rise to the Primates 

 (Lemurs) or to the Carnivores. The order certainly dates from 

 the Secondary Epoch, and one genus — Erinaceus (the Hedge- 

 hog) — is one of the oldest genera of existing mammals, for 

 it has continued on the earth (in France, if not in England) 

 in an unaltered form from the Miocene period of the Tertiary 

 Epoch. 



^ In Centetes, a Madagascar genus, there would appear to be four pairs 

 of true molars. This may also occur in other insectivorous forms. See 

 p. 115. 



2 In several Marsupials there are even five incisors on either side of the 

 upper jaw, and in a great many there are four. On the other hand, in the 

 Theriodont reptiles it is doubtful whether there were more than three on 

 each side. A proto-mammal with four incisors on each side of the upper 

 jaw has been postulated to explain this feature in the Marsupials, and perhaps 

 in the Shrews; but the more reasonable suggestion has been put forward 

 latterly that in the Marsupials the fourth and fifth incisor on each side may 

 be only an instance of the first, or " milk," dentition remaining persistent. 

 Either this may be the explanation of the four incisor teeth on each side of 

 the upper jaw in the Shrews, or the extra incisor may be a canine, and the 

 supposed canine a premolar. 



