PURBECK FORMATIONS. 23 



features that I became satisfied of their relationship to the Mammalian class. I proceed 

 to give results of re-examination of these specimens. 



In a slab of Purbeck shale, showing part, and the impression of more, of a mandible, 

 with teeth, nearly the whole of the hinder half of the ramus is exposed (PI. I, fig. S2, 

 natural size, in outline; 32 a, twice natural size). This contains four teeth, not quite in 

 contact, having a long and rather slender crown, terminated by a sharp-pointed sub- 

 triedral cone (fig. 32 c, o), the inner side of the base of which is produced before and behind 

 the main cone into a short small cusp {e, s). This tricuspid crown is implanted by two 

 roots (fig. 32 b) in a distinct bifurcate socket of the jaw-bone. 



The four teeth in fig. 32 a gradually diminish in size to the hindmost ; the jaw becomes 

 slightly contracted vertically behind the teeth, and then expands to include a smooth depres- 

 sion, recognisable as the fore part of that for the insertion of the crotaphyte muscle ; the upper 

 swollen boundary {b) indicates the fore part of the base of the coronoid process ; the lower 

 boundary ridge (a) is that which would have been continued to the condyle and the 

 angle of the jaw. We have here a left mandibular ramus with the outer side of the 

 preserved bone and teeth exposed. The outer surface is vertically convex ; the inner one, 

 as indicated by the impression, is flatter, and faintly shows the termination of a longitu- 

 dinal channel. 



Fig. 32 B is an oblique view of the anterior side of the crown of the first of the four 

 teeth in fig. 32 a ; it shows the basal ridge of the outer side of the crown, ascending to be lost 

 on the anterior accessory cusp ; fig. 32 c gives the form of the summit of the crown of 

 the tooth viewed vertically. 



In the number, proportion, and relative position of the cusps, this modification of the 

 insectivorous molar resembles that in the Cape Mole {Chrysochloris) ; but in the proportion 

 of the mid-cone (o) and the definition of the side ones it accords more closely with the 

 type of the Amphitherian molar (PI. I, fig. 25 b). 



The inner antero-posterior extent of the crown is considerable as compared with the 

 proportion of that diameter with the height of the crown in the true molars of any of the 

 modern Moles and Shrews, except the ChrysoMoris. The impressions of the inner side 

 of some teeth anterior to those in place show plainly the tricuspid character of the crown, 

 and indicate also a greater number of sach molars in the fossil than in any of the recent 

 Mammalia, with the exception of the marsupial Myrmecohius. Of this further and more 

 important affinity of the Spalacotherium to the AinpJiitherium the following specimens 

 yield more decisive evidence. 



A portion of the marly fresh-water shales from the Purbeck series at Durdlestone 

 Bay, marked ' mammalian beds ' in fig. 4, had imbedded in it part of the lower jaw of 

 the Spalacotherium, wanting the ascending branch ; the alveolar tract includes one incisor, a 

 canine or canine-shaped premolar, and ten succeeding teeth. It is represented of the natural 

 size in outline PL I, fig. 33, and magnified in tint twice the natural size at fig. 33 a. The 

 incisor (^) seemed to be the smallest of these teeth, but is represented only by a portion of the 



