50 FOSSIL MAMMALIA OF THE 



surface sinks as it slightly expands from the produced sharp summit of the inner cusp, 

 and is concave from within outwards : it is very smooth and polished. The series of 

 grinding surfaces of these antero-posteriorly compressed tri-cuspidate molars describes 

 lengthwise a moderate convexity downward or tow^ard the mandible, which curve, with 

 the horizontal outward bend of this part of the dental series, confirms the determination 

 of these teeth, as belonging to the upper jaw. 



The maxillary molars of Styhdon show most resemblance to those of Chri/socJdoris, 

 but they are close together, and, indeed, obliquely overlap each other. Being broadest 

 transversely, and extending from within slightly backward as well as outward, a small 

 part of the hinder surface, as well as the narrow columnar side, of one tooth stands clear 

 of the crown of the following tooth. 



If, as is most probable, one of the minute premolars has dropped from the interspace 

 between the first {p i) and the next [j) 2) in place, we may reckon five premolars 

 according to shape, and seven molars by the same character, twelve in all, on each side 

 the upper jaw, besides the canines and an unknown quantity of small incisors. The 

 indication of premaxillaries («) testifies to a greater extent of dentigerous jaw anterior to 

 the canine than exists in the Cape Mole. 



In the outline of the natural size, PL II, fig. 14 (and in fig. 14 a, magn. nearly four 

 diam. in tint), I conclude, therefore, on the foregoing grounds, is represented a great part 

 of the left upper jaw and dentition of Styhdon pusillus. 



To the same genus and species I refer the left mandibular ramus, with five molars in 

 Hitu, opposed by portions of the crowns of the corresponding upper teeth (PL II, figs. 16 

 nat. size; 16 a, magn. 3 diam.). A few of the premolars are more or less perfectly 

 preserved in the part of the mandible in advance of the lower molars. 



The ascending ramus and the symphysial end of this mandible are broken off; the 

 inner surface of the preserved part with teeth is exposed. The longitudinal groove on 

 the inner side of the ramus is relatively wider than in most other genera of small Mesozoic 

 Marsupials, showing that groove. 



Fragmentary portions of the maxilla to which the portions of upper molars belonged 

 are discernible in the matrix below the mandibular ramus, but too much crushed for 

 working out to any purpose. 



In the subject of fig. 18, PL II, we have acceptable confirmation of the dental 

 characters of the lower jaw shown in figs. 15 and 17. It consists of the two rami of the 

 mandible separated in such a way that the left presents an outside view, and the right 

 turns its lower border to the surface of the slab ; part of its outer surface has been exposed 

 by removal of matrix. 



It is evident that the skull was crushed, together with the jaw, prior to being imbedded 

 in the lacustrine or fluviatile marl. Farts of the basioccipital and basisphcnoid, showing 



