88 O. 0. Marsh — Cretaceous Mammalia. 



shape, this is very similar to some of the smaller jaws of 

 Dryolestes, and the species it represents may be provisionally 

 placed in that genus. A characteristic feature of this specimen 

 is a distinct mylo-hyoid groove, which has essentially the same 

 position as in Dryolestes. This jaw measures 5 mm . in depth, 

 under the molar series, and about 2 mm . in thickness, at the 

 same place. The number and form of the teeth cannot be de- 

 termined from the present specimen. The animal represented 

 was about as large as a mole. It was found in Wyoming. 



Didelphodon vorax, gen. et sp. nov. 



In the present collection of mammalian remains, there are 

 quite a number of teeth, and various parts of the skeleton, in- 

 dicating several species of small animals which appear to be re- 

 lated to the modern opossum. A tooth of one of these is repre- 

 sented in Plate IV, figures 1-3, which may be taken as the type 

 of the genus. 



It is apparently the penultimate upper molar, and has the 

 essential structure of the corresponding tooth in the genus 

 Didelphys. It has, however, a pair of intermediate small 

 cusps near the middle longitudinal line of the tooth, one in 

 front, and one on the posterior border, as shown in figure 2. A 

 number of other teeth, apparently of the same species, were 

 also secured. They are all from the Laramie of Wyoming. 

 They indicate an animal about the size of a rabbit. 



Didelphodon ferox, sp. nov. 



A still larger species, equal in size to the Virginia opossum, 

 is represented also by a number of teeth, of which the lower 

 molar, shown in Plate IV, figure 4, may be considered the 

 type. Its crown has the same general composition as that of 

 the lower molars of the modern Didelphys, but the anterior 

 portion is more elevated. The fangs are especially large and 

 powerful. The canine tooth of destructive form, figured on 

 Plate IV, figures 26-28, probably pertains to this animal. The 

 remains of this species are apparently the largest of any of this 

 class yet found in the Laramie of Wyoming. 



Didelphodon comptus, sp. nov. 



A third species, much smaller than either of the other two, 

 is indicated by various teeth. The lower molar, shown in 

 Plate IV, figures 5, 6, and 7, is a characteristic tooth, and may 

 be taken as the type. The structure of the crown is well 

 shown in the cuts mentioned. There is a well-marked basal 



