286 MR. R. F. TOMES ON A COLLECTION [June 25, 



the back, where the rufous colour is less pure than on the sides of 

 the body. On the outside of all the limbs this colour is considerably 

 tinged with dusky. A narrow black line runs from the roots of the 

 whiskers to the eye, and narrowly fringes the latter. The upper 

 surface of the fore feet is brownish white, of the hinder ones white, 

 with a dusky patch extending along their middle as far as to the 

 roots of the toes, the latter being wholly white. All the under parts 

 are pure white, as are the insides of the fore legs and the thighs. 

 The line of separation of the colours of the upper and under parts 

 is very distinct ; it extends along the cheek to the whiskers, leaving 

 only a narrow band of rufous under the eye. The white fur of the 

 under parts is uniformly white from root to tip ; but that of the 

 upper parts is darkish ash-coloured at the root for two-thirds of its 

 length. 



The skull of this species is of a very peculiar form. It is short 

 and broad, with the nasal part short and compressed, and the 

 zygomatic arches much expanded. At first sight it has much the 

 appearance of the skull of Reithrodon cuniculoides ; but the zygoma 

 is more expanded anteriorly, the nasal region more compressed, and 

 the space between the orbits much wider. That part which is in 

 front of the zygoma is not more than one-fourth of the entire length 

 of the skull ; and of the remaining three-fourths the zygoma takes 

 up two-thirds ; its anterior root springs out at once laterally in such 

 a manner as to be at right angles Avith the nasal bones. The bony 

 plate which descends from this part of the zygoma to enclose the 

 ante-orbital foramen is very narrow, so narrow as to be wholly out 

 of sight when the skull is seen from above, and to give this foramen 

 a forward direction instead of opening obliquely upwards as well as 

 forwards, as in all other species of Hesperomys which I have seen, 

 with the exception of II. latimanus and H. bicolor, which resemble 

 the present one in this respect. The upper surface of the skull pre- 

 sents two strongly marked ridges or crests, diverging from a short 

 distance behind the extremity of the nasal bones to the outer angles 

 of the occipital bone. 



The upper incisors are somewhat compressed, their anterior sur- 

 face rounded, but there is a faintly indicated though quite perceptible 

 ridge along the middle of each, and another similar but more strongly 

 marked one at their inner angle. The molars are parallel with each 

 other, and they decrease much less in size from front to back than 

 is usual. The folds of enamel are more numerous than in most, if 

 not all, other species of Hesperomys *. 



The lower jaw has the corouoid process very small, but very acute, 

 the condyloid one very long, and rendered strong by an external 

 longitudinal ridge. The descending or angular process is a well-de- 



* The number of primitive cusps in this species is what we usually observe 

 but they are penetrated by small folds of enamel near to their summits, and these 

 are wholly lost sight of in -worn teeth. Between these cusps are some small 

 accessory ones, the enamel enclosing ■which forms no part of the exposed section 

 of the molar until it is much worn. Hence a considerable number of these teeth 

 Laving had a different amount of wear are necessary for a proper description. 



