24 = =Myr. E. D. Cope on the Bunothertan Mammalia. 
as appears at first sight to be the case with such animals as 
the Talpide. The intermediate cusps are really such, although 
the posterior looks like the apex of a V-shaped external cusp. 
In Perathertum the external cusps are smaller than in Di- 
delphys, and the intermediate V’s so much better developed 
that the type is much like that of the Talpide, to whose 
neighbhourhood I originally referred it. 
This leads to a consideration of the question of the homo- 
logies of the cusps in the genera of the old order of Insectivora 
proper, and of the Creodonta. Mr. St. George Mivart has 
briefly discussed the question, so far as relates to the former 
group*. He commences 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 V’s, as in the ungulate types. The V’s are 
well developed in several families; and in Chrysochloris the 
two V’s are supposed to be united and to constitute almost the 
entire apex of the crown, while in Centetes the same kind of 
V forms a still larger part of the crown. 
I believe that 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 im- 
probability im the supposition that the 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 different, 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 pro- 
duced by the connexion 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 op- 
posite the internal root. 
First, as regards Centetes and Chrysochloris. Besides the 
strained character of the hypothesis that supposes the V-shaped 
summit of the crown to represent 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 
Creodonta. 
This clearly consists of the two external and the anterior 
internal cusps united by angular ridges. ‘The form is quite 
the same as in Leptictis and Ictops, and nearly that of Delta- 
* Journal of Anatomy and Physiology, il. p. 138, figures. 
