1S99.] 



TLEXODONT MOLARS OF MAMMALS. 



559 



with again iu many Insectivora (Talpa, Tupaidse, Soricidae, ikc), 

 and likewise in the Chiroptera, especially in A^espertilionidse, the 

 most numerous and ancient family. In all these groups the 

 molars differ from those of Proteodide^^hys only by the greater or 

 lesser development of cusp ma, by the suppression oE cusp ai or 

 its fusion with ae, and by the varying degree of simplification of 

 the posterior lobe. 



Fig. 3. 



"y^ a 



Cyonasua argentina : fifth right lower uiolar, superior («) and external {h') 

 aspect, nat. size. — Eocene ; Patagonia. 



Another branch, likewise originating from the most primitive 

 Microbiotheridae, are the diprotodont Marsupials, which comprise 

 the extinct Multituberculata of the IN'orthern Hemisphere and 

 Argentina, the numerous Paucituberculata of South America, and 

 the Diprotodonts of Australia (Hypsiprymnoidea). Their most 

 primitive type is that of the Grarzonidae. The lower molars of 

 Garzonia or Halmariphus (fig. 4) are not distinguished by any 



Halmariphus didelphoides : fifth i-ight lower molar, superior («) and external {h) 

 aspect, eight times nat. size. — Eocene ; Patagonia. 



essential character from those of the Didelphyidye ; their teeth 

 exhibit the six cusps of those of Proteodideljiliys. with an almost 

 similar disposition and with the same external cingulum, c. Some 

 species depart slightly from this form by the internal displacement 

 of the two median cusps, the anterior and the posterior, so that 

 each molar presents on the internal margin a range of four cusps, 

 as can be seen in the molars of a Cretaceous species of Halmariphus, 

 or a nearly related genus (fig. 5). In the Epanorthidge the paired 

 cusps ae, ai, and pe, pi, are connected, forming two semicircular 

 crests. In the Abderitidse the same cusps constitute two feebly 

 accentuated, transverse crests. The slightly more recent Diproto- 

 donts of the Parana deposits (Zi/golestes) exhibit the same crests 

 more accentuated ; they are still more developed in the exisrinj; 

 South-American genus CamolesUs of O. Thomas, the molars of 

 which have assumed the same form as those of the Australian 



