1892.] NATURAL SCIENCES OF PHILADELPHIA. 423 



and tritocones, the tooth cannot be called niolariform. Ij is even 

 more comi3licated than the molars. As in the latter the two exter- 

 nal cusps are of equal size, and the deuterocone is much extended 

 transversely, also a small tetartocone corresponding to the hypocoue 

 of the molars is present. In addition to these parts there is a small 

 basal cusp at the antero-external angle of the crown which does not 

 occur in the molars but which is found on Pi of many creodonts 

 and carnivores. In the lower jaw j? andVare small and simple, the 

 former being carried by one root and the latter by two : this tooth 

 is larger than the corresponding upper one. T^ has added a small 

 metacouid. T* is molariform, with the metaconid enlarged to a 

 talon, and para- and deuteroconids added. This tooth is distin- 

 guished from a true molar only by the greater relative size of the 

 protoconid. The White River species of this genus, J. dakotensis, 

 differs from the more ancient one, with respect to the dentition, 

 only in the fact that the deuterocone of P^ is no longer a conical 

 cusp, but transversely extended and crescentoid, the proto- and 

 tritocones are also more nearly equal in size ; in other words the 

 tooth is now completely molariform, even to a rudiment of the 

 tetartocone. 



In the White River genus, J/e-sof/ecfe.SjP^: is molariform, but Pa has 

 no tritocone, the tooth consisting of a compressed and trenchant pro- 

 tocone, and internal to this a conical deuterocone. Cope calls these 

 teeth P4 and ^ respectively, but this I think is a mistake. At all 

 events the statement, that, " in Leptictls the last premolar is sectorial 

 in form consisting of a single compressed longitudinal crest without 

 internal tuberosity or cusp, " is incorrect. (No. 2, p. 801.) 



In the genus Leptictls, also from the White River beds, p., is 

 without the deuterocone and resemblesthe anterior ^^remolars in every 

 respect, except that it is slightly larger. £4 is molariform, and Pi^ 

 differs from that of Ictops in being inserted by two fangs. So far 

 as we can judge at present, the series Ictops, Mesodectes, Leptictis, 

 would appear to represent successive stages in the simplification of 

 Ps. Doubtless the same is true of the lower premolars, but the 

 mandibular dentition of Mesodectes and Leptictis is unknown. 



Somewhat similar facts may be observed in comparing some of 

 the extinct European iusectivores with their nearest living allies. 

 Thus Schlosser (No. 10. p. 117) regards the genus Parasorex as in 

 some measure connecting the Tupaiidce and the Macroscelid(B, but 

 the extinct form has more complex premolars than either of the 



