224 ON THE EXTINCT MAMMALIA OF 



external to tfie middle of its course, and successively deepens in the three molars at 

 its outer closed extremity. The antero-internal lobe bulges about its middle into the 

 principal valley, but the bulging successively declines in the series from first to last. 



The last true molar is totally devoid of a postero-internal lobe, its position being 

 occupied by the increased obliquity of the outer portion of the crown. 



In the three large upper premolars of R. occidentalis the basal ridge is better devel- 

 oped than in the true molars. It is, however, obsolete at the bottom of the antero- 

 external lobe. It is continuous around the inner half of the crown, and is thickest 

 at the inner back part of the postero-internal lobe. The internal lobes of the three 

 teeth under consideration are confluent at their inner expanded extremities, but least 

 so in the first of the series. In consequence of this confluence, in the third and fourth 

 premolars, of the specimen under examination, trituration has converted the prin- 

 cipal valley into an enamelled pit, opening upon the masticating surface. 



In the first premolar of the specimen the internal lobes of the crown are distinct, 

 and the principal valley remains open. 



The inferior molar teeth of 72. occidentalis are like those of all other species of the 

 genus. They have oblong quadrilateral crowns, composed of a pair of cuboidal lobes 

 excavated in a gouge-like mauner an tero-in tern ally, and having, in the unworn con- 

 dition, acute, somewhat rectangular, crescentoid summits. In the process of tritura- 

 tion, the acute summits of the lobes are worn away and tracts of dentine become 

 exposed, which gradually widen and become confluent on each pair of lobes composing 

 the crown of a tooth. 



Constituent portions of a basal ridge festoon the lower molars of R. occidentalis, and 

 are best developed on the premolars. 



In the specimen of the lower jaw of Prof. Hall's collection there are no traces of 

 the existence of a first pi'emolar, but in another specimen of Dr. Evans' collection, 

 belonging to a younger and considerably larger individual, a connate pair of fangs, of 

 a small tooth, are preserved. 



Of incisor teeth of Rhinoceros occidentalis, the lower jaw of Prof. Hall's collection 

 contains those of both sides, as represented in plate XXI, figure 34. They are four 

 in number as in the Indian Rhinoceros, are comparatively small, but resemble those 

 of the latter animal in form. 



The lower lateral incisor has a compressed cylindrical fang with a pyramidal 

 crown, measuring at the base seven and a half lines wide and five and a quarter lines 

 thick. Externally the crown is convex, broad and flattened in front, and internally 

 is worn off in a sloping manner to the base. 



The lower internal incisor has a rather ovoidal crown, strongly convex in front, 

 slightly so behind, and is bounded by a basal ridge forming a single festoon. The 

 crown is only four and a half lines wide transversely and three and a half fore 

 and aft. 



