Vol. 70.] VERTEBRATES FROM BRITISH EAST AFRICA. 169 



HYKACOIDEA. 



One of the most interesting of the specimens found is a fragment 

 of the left ramus of the mandible (PI. XXVIII, figs. 4>a & 4?b) 

 of a small mammal about as big as a large rat. This is from Bed 16 

 at Kachuku. At first sight the jaw appears to be that of a Rodent, 

 the great hypsodonty of the molars lending support to this view ; 

 but further examination shows that it cannot possibly belong to a 

 member of that order, on account of the form and arrangement 

 of the premolars. The jaw itself is gently convex from above 

 downwards externally, and nearly flat internally ; it is broken off 

 immediately behind the symphysial region in front, and probably 

 about the middle of the molar series behind — from before back 

 wards it deepens with a gentle curve. 



Four teeth are still in situ ; in front of these are the two root? 

 of another tooth, and anterior to this again an imperfect alveolus 

 all these teeth formed a closed linear series. Anterior to and 

 below 7 this is the base of a larger tooth, the section of which is 

 an elongated oval with the long diameter transverse ; it appears to 

 have been directed forwards and upwards, as if it were the root 

 of a large procumbent incisor. Beneath it is a cavity which may 

 represent the alveolus of another tooth, but this is very doubtful. 



The first of the teeth preserved is doubtless a premolar 

 (PL XXVIII, fig. 4 5, pm 3 ). ; its crown is composed of two 

 elongated U-shaped lobes, separated externally by a short deep 

 groove, which does not extend to the base of the crown. On 

 the inner face the lobes are not so clearly separated, the face 

 being nearly flat in consequence of wear. The anterior lobe is 

 the bigger, and is borne on a large root ; while the smaller 

 posterior lobe has a much smaller root. The enamel is quite 

 smooth, and is thickest on the outer face of the tooth ; there 

 is no trace of a cingulum. The tooth in front was smaller, but 

 doubtless otherwise similar, the roots showing exactly the same 

 arrangement. 



The crown of the next tooth preserved is also composed of two 

 elongated iobes, somewhat V-shaped on the outer face, the anterior 

 arm of the V being much the longer. Externally, the lobes are 

 separated by a deep vertical groove extending nearly to the base 

 of the crown, which is here becoming very high and prismatic 

 in form ; the tooth, as a whole, is slightly curved, with the convexity 

 directed forward. On the inner face the columns are likewise sepa- 

 rated by a vertical groove : the anterior column is the lono'er from 

 before backwards ; its postero - internal edge forms a prominent 

 ridge i metaconid) on the inner side of the tooth, constituting the 

 anterior lip of the vertical groove already mentioned. 



The next tooth is still more hypsodont, and the columns are 

 stouter and broader, their crescentic form being less distinctly 

 shown. The same is also true of the next tooth, in which the 

 hypsodonty reaches its highest pitch, so that there is considerable 

 resemblance to some Rodent molars. Despite the hypsodonty, 



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