OCR Text |
Show 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 of H. 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 H. 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 truncated 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 in H. phillipsi 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 recorded1, 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 of Heterocephalus as yet examined, nor can H. phillipsi, in which there are only two molars, be said to be as highly specialized 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 Eeise n. Mossamb., Saug. p. 142, 1852. 55* |