OF DIDELPHYS VIRGINIANA. 49 



between the second and third under premoLars, its apex resting npon the basal snag of 

 the former ; the third under premolar is similarly wedged between the second and third 

 upper ones ; the latter, again, fits in the interspace between the third under premolar and 

 first molar, with its apex touching the basal snag of the former. Each of tlie premolars 

 has two long, stout, straiglit roots, parallel with each other ; they cause a deep sulcus to 

 appear on botli inner and outer aspects of the exposed portion of the tooth, before unit- 

 ing to form the single, compressed, conical cusp. The anterior border is a little convex ; 

 the posterior twice concave, as a result of the projection of the basal snag. 



The four molars are remarkable for possessing but two roots apiece, like the premolars, 

 instead of three as those of the upper jaw have. In the form.ation of the crowns, how- 

 ever, the transition from the premolars is abrupt and decided. The roots are straight, 

 slender, terete, tapering, diverging but little from each other, following each other in the 

 line of the axis of the jaw-bone, and are two or three times as long as the exposed por- 

 tion of the tooth. The contour of the crown corresponds to the absence of a third 

 (inner) root; it is a narrow rectangle, without angular extension inward. While quite as 

 long as the upper ones, each for each, the lower molars have not half the ])readth of the 

 latter; when tlie two sets are apposed in the closed jaw, a considerable part of the grind- 

 ing surface of the upper molar lies external to the under series. The molars progres- 

 sively increase in length, width, heighth and prominence of the several irregularities of 

 surftice, from first to fourth. The antero-external corner of each presents a concave 

 depression for the reception of the cusp upon the posterior border of the corresponding 

 upper tooth ; a larger excavation upon the face of the crown, at its back part, similarly 

 receives the prominent inner corner of an upper molar. Between these two depressions 

 the outer border of each molar rises to form a prominent conical cusp, attaining the great- 

 est elevation on the last of the series. Behind the cusp is a smaller one, at the postero- 

 external angle of tlie tooth ; the postero-internal corner makes a still less prominent cusp. 

 The antero-internal angle rises highest of any ; its strong cusp is partially divided in two 

 by a notch. (For under teeth see figs. 7 and 8, page 62.) 



The upper molars have much more of the character of true "grinders" than the lower; 

 the latter arc evidently better fitted for the laceration of animal tissue than for the crush- 

 ing of vegetable fibre. The crowns of the upper molars chiefly present obtuse rounded 

 inequalities of surface ; those of the lower show conical sharp-pointed projections. 

 * The whole series of lower teeth is twisted upon itself, and traverses the jaw obliquely, 

 as shown in fig. 8. 



PART I.— THE BONES. 



1. The Skull. 



The cranium is to be considered under three different aspects. First, as being a phase 

 of the mammalian modification of what has been called the vertebrate archetype ; repre- 

 senting, as such, four vertebra, the elements of which have been subjected to changes of 

 relative size and shape, and also, in many instances, to dismemberment or displacement, 

 and the essential characters of which have been further modified — not to say obscured — 

 by the addition or intercalation of accessory and appendiculate parts, as well as by the 



KKM0IB8 nOST. 80C. NAT. HIST. VOL. H. 18 



