1^56.] OWEN-^STEREOGNATHUS OOLITICUS. 3 



The fore part ef the base of the hinder cone (fig. 5, p^) is prolon«>ed 

 obliquely towards the centre of the crown, beyond the contiguous 

 end of the base of the front cone, jh so as to cause an arrangement 

 hke that of the two outer cones, o, o': the obliquity of the posterior 

 cone of both the outer and the inner pairs, o' and y, being such that 

 they shghtly converge as they extend forwards. 



In the hindmost tooth, c, the two outer cones are broken off, 

 showing that their common base is di^-ided from the two middle 

 cones by a deeper groove than that which separated the two outer 

 cones from each other. 



Thus the crown of these molars might be described as supporting 

 three parallel antero-posterior ridges ; the outer (fig. 5, o, o') and 

 the inner, p, p\ ridges being each divided by an obUque cleft con- 

 verging forward toward the middle of the tooth ; whilst the middle 

 ridge, h, i, is divided by a curved cleft having its concavity turned 

 forward. 



The more mutilated state of the front tooth (fig. 2, a), of which 

 only the base of the middle ridge of the crown remains, throws no 

 additional light on the modifications of the verv remarkable type of 

 the grinding surface of the mandibular molars of the Stereogiiathus. 



This type of tooth differs from that of all other known recent or 

 extinct Mammals. The nearest approach to it is made by the true 

 molars of some of the extinct Mammals from the most ancient of the 

 tertiary strata, e. g. the Hyracotherium and Hyopotamus ; but, in 

 these genera, only in the molars of the upper jaw. 



The two transverse ridges of the molars of the Manatee are each, 

 when first formed, divided into three tubercles ; but the clefts are 

 so shallow that they are soon worn down, and a transverse strip of 

 dentine bordered by enamel is exposed. In the Stereogiiathus the 

 longitudinal clefts are deeper than the transverse one, and the result 

 of abrasion would be to produce three antero-posterior strips of den- 

 tine, instead of two transverse ones. Nevertheless the temporary 

 sex-cuspid character of the molars of the Manatee is interesting 

 from being coupled with a short or low crown, and with a character 

 of lower jaw, thick and rounded below, like that of the Stereogiia- 

 thus ; but the ramus is proportionally deeper in the Manatee. 



In the last upper molar* of the Rat, before it is much worn, there 

 are two transverse rows of three tubercles, but there is also a, hinder 

 lobe of two tubercles. 



The ante-penultimate and penultimate lower molars of the Hedge- 

 hog (Eriiiaceus) show five or six cusps, but they are small, and only 

 two occur on the same transverse line : the same remark applies to 

 the multicuspid molars of the Shrews, the Galeopitheci, and a few 

 other Insectivora. 



In the upper true molars of the Hyracotherium * we have three 

 pairs of cones, each pair being in the antero-posterior direction of 

 the crown, and there being, consequently, two transverse rows of 



* British Fossil Mammals, p. 419, cuts 165, 166 ; and Geol. Trans. 2 ser. vol. vi. 

 pi. 21. figs. 1, 2. 



b2 



