PROF. OWEN ON PEOEASTOMUS SIEENOIDES. 563 



In the mandible the root and part of the broken crown of an inci- 

 sor, apparently answering to the third above, is retained on the right 

 side, as shown in PL XXIX. fig. 4, % 3, and in figure 2, at i, pi. xv. 

 of the original description*. After an interval of 2\ lines the long 

 fang of the canine is exposed, as shown at c, PI. XXIX. fig. 4. After 

 an interval of 10 lines is the fang of a single-rooted premolar (" first" 

 of the grinding series, ib. fig. 4, d i) ; near to this is a similar root 

 of the "second" (ib. ib. a 2). The " third" low r er premolar is less 

 distinctly shown. In the " fourth," part of the enamelled crown is 

 preserved : it is 3| lines in fore-and-aft extent. The " fifth " 

 premolar has a similar size and shape ; the crown is preserved and 

 is supported on a single fang. 



The " sixth " tooth (first lower molar, m 1) shows the shallow ver- 

 tical indent at the middle of the outer side of the crown ; and two 

 fangs correspond to the two coronal lobes so indicated. The two fangs 

 are also preserved of the " second molar/ m 2 (" seventh " tooth) ; but 

 the crown is gone or hidden. The opposer of the last upper molar 

 appears to have been lost on the right side ; in the left ramus it is 

 in place, and the inner side of the enamelled crown is exposed, 

 showing a slight indication by a shallow vertical groove of its two- 

 lobed structure. The two fangs of this molar, and of the two ante- 

 cedent ones, are also visible in the left mandibular ramus. 



The transverse breadth of crown in the lower molars is less than 

 that in the upper ones ; and they are opposed to the inner half of the 

 grinding surface of the teeth above, and this seemingly to a greater 

 degree than in the Manatee. The enamel is thick, and presents a 

 deep brownish tint in the fossil. 



The dental formula of Prorastomus, according to the indications 

 susceptible of being worked out in the unique fossil, is : — i J-jf, c — 

 d or p ^, m |^|=48. The premolars are here so called on the 

 ground of position, smaller size, and simpler structure. 



The molars are bilophodont, with two outer and two inner roots, at 

 least in the upper jaw. The premolars are tubercular, or with simple 

 crowns and single roots, or partially divided roots, except, perhaps, 

 in the hindmost above, where the vertical division seems to be com- 

 plete. The upper true molars (PL XXIX. fig. 2) have an anterior 

 basal ridge (/) and a posterior oblique partial basal ridge (g); each 

 lobe seems to have been a transverse ridge prior to abrasion. 



In the enamelled covering of the crown, in its low vertical extent 

 as compared with its antero-posterior and transverse breadths, in 

 the two-ridged structure of the true molars, Prorastomus accords 

 with the existing genus Manatus, and differs, with it, from the ge- 

 nera Halicore and Bhytina. 



In the excess of the molar series beyond the number " seven," on 

 each side of both jaws, Prorastomus, like Manatus, departs from the 

 type of the terrestrial vegetable-eaters of the Mammalian class, and 

 gives an interesting indication thereby of a Cetacean character. 

 But the existing Manatus departs further from type, developing ^~ 

 in the molar series instead of |^|, as in Prorastomus. This earlier 



* Tom. cit. p. 543. 



