VERTEBRATE REMAINS, PORT KENNEDY BONE DEPOSIT. 207 



formula is identical with that of M. dideltus, but the trefoil is smaller in that 

 species. In another specimen, represented by incisor and sup. m. 1., the former has 

 an oblique antero-external face, with narrow truncate outer face ; enamel not striate, 

 emarginate at the cutting edge. In M. dideltus (fossil, below), the antero-external 

 face is more oblique, and without defined external plane ; the end is not emarginate 

 (in one specimen). This tooth appears to be relatively smaller and weaker in 

 the ? M. speothen. 



Measurements. 



mm. 

 Length, fang and crown 1st superior molar (no. 3) ; 4 



Width, enamelled face incisor superior (no. 3) ; 1 



Two superior molars of another and rather smaller individual were referred to 

 a distinct species in my original paper on the Wheatley collection under the name 

 of Arvicola tetradelta. I now refer them to this species. In making the drawing 

 they were represented, as they lie in the matrix in reversed position, both the m 1 

 and m 2 - being turned about. They do not differ from the corresponding teeth of 

 M. speothen, the anterior triangle being as in it well formed and entirely distinct. 



MlCROTUS DIDELTUS Cope. 



Arvicola didelta, Proc. Am. Phil. Soc, 1871, p. 89, fig. 15. 

 Arvicola sigmodus Cope, /. c. , p. 90, fig. 1 7. 



Represented by the mandibular rami of five, and superior dentition of probably 

 two individuals. One imperfect cranium contains the dentition of both jaws, thus 

 fixing the relations of fragmentary specimens, especially in the more important 

 relation of the anterior, inferior and posterior superior molars. The characters of 

 these show that it is allied to M. pinetorum. 



The triangle formula derivable from the material is as follows : Superior : 

 (1) If ; (2) 1} 1; (3) 1^ + strongly sagittate trefoil; inferior: (1) trefoil with 

 strong basal angles -f-1 ; (2) 1^ 1 ; (3) 1-1-1. The trefoil of the m 1 is large, elon- 

 gate, subsymmetrical and rounded in front. At its base on each side is a prominent 

 angle representing a pair of triangles, which are, however, not closed in front, 

 although one of them is sometimes nearly so. Anterior to these is a low angle on 

 each side, whose distinctness varies with the individual. In the last superior molar 

 there is on each side a well-defined and isolated triangle, followed by a trefoil with 

 a prominent reflexed angle on each side at the base, giving it an obtuse sagittiform 

 outline. In the second inferior molar the anterior triangles of opposite sides are 

 not isolated from each other, but they are well indicated by an indentation of the 

 anterior border. In the nig- the triangles of opposite sides are fused together form- 

 ing three transverse lenses. 



Comparison with M. pinetorum is facilitated by Mr. G. S. Miller's figures and 

 descriptions already referred to. The m T is quite similar in the two species. The 

 triangles of the m^ are not nearly so well defined in M. pinetorum, nor are the 

 processes at the base of the loop so large and acute angled. In the m- 2 - the anterior 

 triangles of opposite sides are not as well outlined as in M. dideltus. 



