1883.] NATURAL SCIENCES OF PHILADELPHIA. , 81 



better developed, that the type is much like that of the Talpidae, 

 in whose neighborhood I originally referred it. 



This leads to a consideration of the question of the homologies 

 of the cusps in the genera of the old order of Insectivora proper, 

 and of the Greodonta. Mr. St. George Mivart has briefly discussed 

 the question, so far as relates to the former group.^ He com- 

 mences with the primitive quadrituberculate type presented by 

 Gymnura and Erinaceus, and believes that the external cusps 

 occupy a successively more and more internal position till they 

 come to be represented by the apices of well developed T's, as in 

 the ungulate types. The V's are well developed in several 

 families, and in Ghrysochloris the two V's are supposed to be 

 united and to constitute almost the entire apex of the crown, 

 while in Gentetes the same kind of a V forms a still larger part 

 of the crown. 



I believe thnt these conclusions must be modified, in the light 

 of the characters of various extinct genera, and of the genus 

 Didelphys. In the first place there is an inherent improbability 

 in the supposition that tlie external V's of the superior molars of 

 the Insectivora have had the same origin as those of the Ungulata. 

 The movements of the jaws in the two groups are diflerent, the 

 one being vertical, the other partially lateral. In the one, acute 

 apices are demanded ; in the other, grinding faces and edges. We 

 have corresponding V's in the inferior dental series, and we 

 regard those as produced by the connection of alternating cusps 

 by oblique ridges. In homologizing the superior cusps, we have 

 as elements, two external, two intermediate, and two internal 

 cusps. The first are opposite the external roots, and the anterior 

 internal is opposite the internal root. 



First, as regards Genletes and Ghrysochloris. Besides the 

 strained character of the h} pothesis that supposes the V-shaped 

 summit of the crown to repi-esent two V's fused together, there is 

 good evidence obtainable in support of the belief that the triangle 

 in question is the usual one presented by the Greodonta. 



This clearly consists of the two external and the anterior 

 internal cusps united by angular ridges. The form is quite the 

 same as in Lepticti's and Mops, and nearly that of JJeliatherium, 

 where the external cusps are present. Gentetes and Ghrysochloris 

 only differ from these in that the external cusps are wanting. In 



Journal of Anatomy and Physiology, ii, 188, figures. 



