1885.] RODENT GENUS HETEROCEPHALUS. 847 



four pairs of small, rounded elevations ; but they are so vaguely 

 defined that their exact number is not easily determinable. 



The skull oi U. phillipsi (Plate LIV. fig. 4) is smooth and rounded, 

 short in proportion to its size, with a broad flat brain-case and a 

 very broad interorbital region. Compared to that of H. glaber its 

 most striking characteristic is its very much smaller size, as is 

 shown on the Plate (figs. 4 and 5), where the two skulls are drawn 

 on the same scale. This difference in size is so marked that it is 

 obvious at the first glance that the owners of these two skulls could 

 not possibly belong to the same species, notwithstanding their extreme 

 resemblance to each other externally. 



In their general proportions also the two skulls differ noticeably, 

 the facial portion of that of JS". phillipsi being much shorter, in fact 

 only about three quarters of the length of the brain-case, while in 

 H. glaber the lengths of the face and brain-case are about equal. 



The nasals of H. phillipsi are short and somewhat squarely trun- 

 cated behind, and are surpassed posteriorly by the ascending 

 processes of the premaxillae, while in H. glaber they are more 

 pointed behind, and are about equal in length to the premaxillary 

 processes. 



The anterior part of the zygomata, opposite the postorbital 

 processes, is much more bowed out in H. phillipsi than in H. glaber. 



On the underside of the skull the only difference appreciable is 

 that the palatine foramina, minute in both, are still smaller m 

 H. phillijisi than in H. glaber. 



Passing to the teeth, we find a very remarkable distinction between 

 the two animals. In H. glaber there are three round and simple 

 molars in each jaw ; but in H. phillipsi there are only two, both 

 above and below, the tooth absent being apparently the first. In 

 any other family this difference would be of generic importance ; but 

 in the present group analogous differences occur even in the same 

 species, as for example in Heliophobius argenteo-cinereus, Peters, 

 which, as its describer has recorded \ sometimes has two and 

 sometimes three premolars. And, again, Georychus capensis, Pall., 

 has sometimes one and sometimes no premolar. For the present 

 therefore too much stress must not be laid upon the difference 

 between the only two specimens oi Heterocephalus as yet examined, 

 nor can H. phillipsi, in which there are only two molars, be said to 

 be as highly sjiccialized in this respect as Hydromys, otherwise the 

 only Rodent with as few molars in each jaw. H. phillipsi has, in 

 fact, no doubt, as a rule, the same number of molars as H. glaber, 

 even if both do not sometimes have either one or two premolars 

 developed in addition in front of the molars. 



The teeth themselves are rounded and very simple, having each 

 but one single external fold of enamel, which seems to disappear as 

 time advances, as the specimen of H. phillipsi, apparently the more 

 aged of the two, shows scarcely a trace even of this fold. The 

 lower molars, at least of H. phillipsi, have each one external and one 

 internal fold, and from Riippell's description those of H, glaber are 

 1 Beise n. Mossamb., Saug. p. 142, 1852. 



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