EHINOCEKOS HEMIT(ECHUS. 327 



of the middle valley nearly effaced. The central fissure 

 forms a very irregular chasm deeply indented by the salient 

 processes of a bifid ' crochet ' thrown off in front_ of the 

 posterior fossette, and by a thick projecting plate given off 

 from the middle of the longitudinal outer ridge and con- 

 verging towards the top of the crochet. If during the 

 further progress of wear the points were to run together into 

 a common surface, a third detached fossette would be formed, 

 exactly as is seen in the preceding specimen, and the anterior 

 border of the posterior colline would present only a slight 

 amount of undulation, instead of the numerous salient plates 

 or denticulations yielded in its present state. These pro- 

 cesses are less conspicuous in the penultimate premolar 

 (p.m. 3), in consequence of its more advanced stage of wear, 

 which has led to their disappearance ; but the two lobes of 

 the bifid ' crochet ' are distinctly discernible in the latter tooth. 



A third condition of the last premolar is furnished by the 

 anterior tooth (p.m. 4) of PI. XVI. fig. 2. Here the abra- 

 sion of the crown has proceeded so far that the transverse 

 valley is reduced to a diagonal excavation, oblong in form, 

 with rounded ends and parallel sides. The enamel boundary 

 of this fossette is perfectly smooth and equal, the projecting 

 processes of the bifid ' crochet ' having entirely disappeared ; 

 and the posterior valley is reduced to a small round pit. On 

 the inner side the waste of the crown by grinding has gone 

 so far that no indication remains of its having been origin- 

 ally composed of two distinct barrels. 



These three examples furnish an instructive series of 

 illustrations of the very different patterns which may be 

 presented in this species by the same tooth in different 

 stages of abrasion. In each case the tooth is fortunately 

 in place in the jaw in connection with other molars, which 

 determine its rank and numerical position with certainty. 

 Had they been found detached it would have been but con- 

 jectural to identify p.m. 4 of fig. 2 (PI. XVI.) with the com- 

 plex crown of p.m. 4 in fig. 1 (PI. XVI.). 



I have seen other detached premolars of R. liemitcechus 

 from various localities, all presenting the same characters, 

 that is to say, the hind barrel projecting into the central 

 fossette a bifid 'crochet,' and an accessory parallel plate 

 emitted from the middle of the outer or longitudinal ridge, 

 forming together three ' combing plates ' of a complex 

 pattern, as in p.m. 4 of fig. 1 (PI. XVI.). Two specimens of 

 this nature, from the cavernous fissure of Durdham Down, 

 are preserved in the Bristol Museum. They are contiguous 

 premolars of the upper jaw, left side. 1 



1 Sco Appendix, No. II., p. 350. — [En.] 



