THE EXTERNAL SKELETON, 



265 



equal in vertical extent the four normal cusps, as in the 

 Shrew-mice (Sorex). 



A prominence may also be developed from the internal 

 part of the band. When this, together with all the other pro- 

 minences, is sharp-pointed (as in the Flying Lemur and the 

 Mole Urotrichus), it produces a maximum of complexity of 

 this special kind. 



UROTRICHUS 



GALEOPITHtCUa 



FIG. 237. GRINDING SURFACES OF UPPER MOLARS OF LEFT SIDE. 



a, b, c, and d, principal cusps ; i, 2, 3, and 4, accessory cusps of external 



cingulum. 

 The prominence of the internal cingulum appears below and between b and d. 



Ridges may be developed to connect the two external 

 normal cusps with the adjacent cusps of the band, thus form- 

 ing two triangular prisms placed side by side, and each with 

 a flat side turned outwards and an angle inwards ; while some- 

 times but a single prominence may represent the coalesced 

 external normal pair of cusps, all of the cusps being drawn 

 out into sharp points suitable for cracking the hard coats 

 of insects, and constituting the typical insectivorous type of 

 molar as seen in the Mole and Bat, and also in the In- 

 sectivorous Marsupials, Pcrameles. 



FIG. 238. GRINDING SURFACE OF UPPER MOLAR OF LEFT SIDE. 

 a, b, and c, principal cusps ; i, 2, 3, and 4, cusps of cingulum. 



A singular modification may be presented by the squeezing 

 together, as it were, from behind forwards of the tooth till it 



