108 EVOLUTION OF MAMMALIAN MOLAR TEETH 



cuspidate condition. As described and figured by Thomas, 1 the posterior 

 superior molars exhibit two high conical internal cusps, from which 

 minute ridges run downward to the external borders of the crowns, the 

 edges of which are peculiarly crenulate rather than cuspidate, in the 

 ordinary sense of the word. In the posterior ////<//>>/ molar there are two 

 high cusps on the external side connected by transverse ridges, with a 

 series of crenulations on the internal side. 



It is especially noteworthy (1) that unlike the Multituberculates the 

 lower molars reverse the pattern of the upper molars (as in tritubercular 

 teeth generally) and (2) that the highest cusps are on the inner side of the 

 upper molars and on the outer side of the lower molars. So far as these 

 facts are of rulni' they would support fltc hypothesis that these are degenerate 

 tritubercular teeth. 



MAESUPIALIA. 



It was early perceived by Cope that the molars of Didelphys (Figs. 

 57, 58 c) are of the tritubercular type, very similar, in fact, to those of 

 the early Eocene Creodonts. We are indebted to Dr. B. Arthur Bensley- 

 for a careful study of the molar teeth of Marsupials in general and an 

 exposition of the very striking parallels which they present to the evolu- 

 tion of the molar teeth in Placentals. In fact, on the supposition that 



FIG. 57. Skull and dentition of the common Opossum (Didilphys i-ii-i/iiiinint) ', a typical Poly- 

 protodont Marsupial, with the dental formula Inc. f , Can. i, Pms. f, Ms. f or 50 teeth in all. 

 x . After Matthew. 



the Marsupials separated at a very early period from the Placentals and 

 that subsequently the dental as well as the general evolution of the two 

 groups was entirely separate and independent, the teeth exhibit some of 

 the most convincing proofs of parallel or homoplastic evolution of which 

 we know. 



1 Thomas, O., ''On the Dentition of Ornithorhynckus," Proc. Roy. Soc., Vol. XLVI. , 

 1889, pp. 126-131, PI. 2. 



- Amer. Naturalist, Vol. XXXV., 1001, p. 251; also Tran*. : -Linn. Sor.,- London, 2nd 

 Ser., "Zool.," Vol. IX., Pt. 3, Dec. 1903. 



