TEETH OF DIPHYODONTS. 309 



The true molars of the tropical Hedgehogs, forming the sub- 

 genera Echinops and Ericulus, are more simple, and approach the 

 form of those in the Chrysochlore, being compressed from before 

 backward, with two outer cusps and one inner cusp in the upper 

 jaw, and with one outer and two inner cusps in the lower 

 jaw. The number of incisors is {•:■§ in both subgenera, which are 

 followed by j*| small and simple premolars ; but Ericulus has J-:f 

 compressed tricuspid molars, and Echinops only fc\. 



The large Tenrecs or tailless Hedgehogs of Madagascar, com- 

 bine the simple molars of Ericulus with the most formidably 

 developed canines which are to be met with in the whole order 

 Insectivora. The incisors are two in number in the upper jaw, 

 but three in the lower jaw; very small and sub-equal in both; 

 the canines are long and large, compressed, trenchant, sharp- 

 pointed, recurved, and single-fanged, thus presenting all the 

 typical characters of those teeth in the Carnivora. They are 

 separated in both jaws by a wide space from the premolars ; the 

 first above is compressed, unicuspid with a hinder talon, and 

 two-fanged ; the second has a larger prismatic tricuspid crown and 

 three fangs ; of the four posterior teeth, which by their antero- 

 posterior compression may be regarded as true molars, the first 

 three have tricuspid crowns as in the Echinops, and have three 

 fangs ; the fourth is smaller, is tricuspid, and has two fangs ; all 

 the lower molars have two fangs. 



The teeth of Insectivora consist of a basis of hard dentine, 

 with a thick coronal investment of enamel, and an outer covering 

 of cement, very recognisable in the interspaces of the coronal 

 cusps in microscopic sections of the molars of the larger species, 

 as the Tenrecs and Macroscelids, and always thick when it closes 

 the extremity of the fangs. Here the cement is commonly more 

 highly organised, is traversed by medullary canals, generally 

 presenting concentric walls ; it thus assumes the character of true 

 bone, and, in the Soricida, is frequently continued into the sub- 

 stance of the jaw itself. 



The small proportion of dentine, in comparison with the thick 

 layer of enamel, has been already alluded to in the Shrews, yet 

 the dentinal tubuli are at their commencement very little inferior 

 in diameter to those of the human incisors ; the trunks are very 

 short, and are resolved into radiated penicilli of undulating 

 branches, which quickly subdivide, interlace and anastomose 

 together near the boundary line between the dentine and enamel. 

 In most of the Insectivora, the secondary branches of the den- 

 tinal tubes are unusually conspicuous, especially in the dentine 



