PURBECK FORMATIONS. 61 



inferiority as compared with the type-jaw (PI. Ill, fig. 7) and the subject of fig. 8. 

 But both this difference of size and some slight modification of the shape of the 

 cones, as viewed from the inner side (fig. 9 a) and outer side (fig. 9 a), may well come 

 within the sexual and other variations of a species. The limitation of the cingulum to 

 the inner siuface of the base of the molars is generic. In subsequent specimens I shall 

 show the reverse position of it in the upper molars of Triconodon. 



The next specimen (PI. Ill, fig. 10, nat. size, and 10 a, teeth, magn. 3 diam.) yields 

 confirmatory evidence of the characters of j5 4 and m i, as shown in the type-specimen 

 (fig. 7). It consists of a right mandibular ramus, with the outer side exposed, wanting part 

 of the ascending branch, of which the form is indicated by a smooth impression on the 

 matrix. 



The alveolar tract includes the last premolar (p 4) and first true molar {m 1), in situ; 

 it shows the empty sockets of the second and third molars, and of a third and second pre- 

 molar, each of which is two-rooted. The socket of the first premolar retains the base of the 

 crown of that tooth {p 1) ; it is preceded by a larger socket of a single-rooted canine, and 

 by the sockets of three small incisors. 



The fore part of the depression for the insertion of the crotaphyte muscle is shown on 

 the part of the ascending ramus preserved. The impression on the matrix shows that the 

 angle of the jaw was inflected, in a way and degree from which may be inferred the marsupial 

 nature of the species. 



The true molar (fig. 10 a, m 1) consists of three principal lobes and a small posterior 

 talon. Each lobe is a subconi pressed sharp-pointed cusp: the mid one rather the largest 

 and highest, but the apices of the three rise nearly to the same level. The ' talon' is also 

 pointed, and ends about half way up the third lobe. The fore-and-aft extent of this tooth 

 much exceeds either its height or breadth, the latter being the least dimension. No cin- 

 gidum appears on the side of the tooth exposed. The crown is implanted by two roots. 



The crown of the last premolar has one conical, subcompressed, pointed lobe, with 

 a minute basal cusp in front and a larger one behind ; the fore-and-aft extent of this pre- 

 molar is nearly equal that of the succeeding three-coned molar. It is implanted by two 

 roots. The size of the first premolar {p 1), as indicated by its base, is less than half that 

 of the last {p 4). The canine must have had an antero-posterior breadth of base nearly 

 equal to that of the last premolar. The incisors were the least of all the teeth. There was 

 no break, or 'diastema,' in the dental series. 



Along the outer side of the ramus near its lower border runs a linear fissure, but this 

 may be accidental ; above the back part of this fissure appears a shallow longitudinal 

 depression, as in the type-jaw (fig. 7). 



As compared with Phascolotheriuni^ from Stonesfield Oolite, with the same dental 



' ' History of British Fossil Mammals,' 8vo, 1846, p. 61, fig. 20. 



