OF THE MESOZOIC MAMMALIA. 
207 
the anterioi and posterior borders of the basin between these cusps are raised into 
prominent basal cusps c and cl; there is also a posterior cusp at the base of the crown e. 
The internal cusps, 6, are broad in the anterior molars and become more pointed pos- 
termrly ; the reverse order is true of the external cusps, a. A pecidiar feature of the 
seiies is the overlapping of the posterior by the anterior cusps. These molars resemble 
the third lower molar of Didelphys, but lack the external basal cusp of the latter. 
The relations of Peralestes and Peraspalax. — The writer’s grounds for not fol- 
lowing Prof Owen in referring the Phascolestes mandible to Peralestes, have already 
been given (jvide Phascolestes). In the jaw of Peraspalax, however, are lower 
molars which closely correspond to what we should expect to find in opposition to the 
Peralestes upper molars. These genera are the only ones among the British mesozoic 
mammals, as yet discovered, with fully opposed cuspsf Not only so, but the molar 
crowns in both genera reverse the usual arrangement of the higher and lower cusps, 
i. e., in the upper molars {Peralestes) the highest cusps are internal and retroA^erted ; 
in the lower molars {Peraspcdax) the highest cusps are external and antero-verted. 
It would appear from this that in each the arrangement of higher and lower cusps 
is as in Didelphys and Sarcophilus. The type maxilla and mandible also agree 
approximately in size. So far as the pattern of the molars and premolars is con- 
cerned there is, therefore, every reason to believe that these specimens belong to the 
same genus, but the number of the molars and premolars presents a difficulty. The 
series are unfortunately incomplete in both specimens, and we have provisionally at- 
tributed these formulae : pm 5, m 6, Peralestes, pm 4, 7; Peraspalax, the whole number 
agreeing in each case, but the division differing. It is, hoAvever, possible that the 
interval between pm^ and the last premolar in Peralestes has been widened by frac- 
ture, and that but one premolar Avas present there; or, that the space behind in 
Peraspalax, held a small premolar ; or that the upper and loAver formulm differed ; 
either alternative Avould enable us to unite the,se genera Avithout further hesitation. 
As the matter noAv stands the safer course is to keep them separate. 
STYLODON, Plate IX, flg. 14. 
The genus Stylodon is knoAvn from numerous mandibular fragments, undoubtedly 
representing more than one species; of these L. pusillus {m. 47,757) is here selected 
and figured as a type. In this genus Ave find a Avholly different type of dentition 
from those Ave have been considering. 
The JaAv is A^ery slender and tapers to the symphysis; there are tAvo mental 
foramina beneath the anterior premolars and the canine. The contour of the coronoid 
process, judging from a matrix impression, is Avide and high, and enough of the pos- 
terior border and angle is preserved to shoAv that the condyle was beloAv the molar 
level. The incisors in L. pusillus are very procumbent. The median incisor is much 
* Leaving out of view, of course, Plagiaulax and Bolodon as belonging to a widely different phylum. 
