OI'.JKCTIONS AND DIFFICULTIES AND OTHER THEOKIKS -J 1 "> 



to sexti-tubercular) Insectivora favors the view that the high internal 

 cusp (protocone) in the former is homologous with the low antero- 

 internal cus}) of the latter (Figs. 04-80).* 5. Since all the orders in 

 which the paracone appears first (Dilambdodont Ensectivores, Carnivores, 

 Primates, Perissodactyls and other Ungulates) are derived from Eocene 

 forms in which the inner side of the crown is depressed, there has been 

 lli'nf;i of time for embryogeuy to have become adapted to this condition 

 and for the main axis of the developing tooth germ to have become 

 shifted to the outer side (p. 54). G. The high protocone of trituberculate 

 Insectivores and the depressed ledge-like protocone of quadrituberculate 

 Insectivores alike fit into the talonid of the lower molars, and thus 

 both function like the protocone of mammals in general (Gregory).t 



The Cope-Osborn theory, however, has to meet three further sets 

 of objections : 



First, those derived from the comparison of the premolars and molars 

 clearly set forth by Wortman (see pp. 195, 142, 210). 



Second, those derived from a restudy of the teeth of Jurassic 

 mammals themselves, developed by J. W. Gidley (p. 219). 



Third, further comparison of the teeth of Insectivores, Chiroptera, 

 and other orders, developed by Gidley (see p. 124). 



2. THE PREMULAI; ANALOGY THEOIIV. 



The theory that the superior molars originally acquired trituberculy 

 in a manner similar to that which can be traced in tin: premolar meta- 

 morphosis has been designated in the introduction as the ' premolar 

 analogy theory ' (pp. 6, 7). 



Premolar evolution, as the key to molar evolution, was suggested in 

 1880 by Huxley, in the following passage: 1 "The exact correspond- 

 ence in plan of these teeth [of Otocyon~] is the more interesting, since, in 

 Gentries, it is easy to trace the successive changes by which the simple 

 and primitive character of the Mammalian cheek-tooth exhibited by the 

 most anterior pnumolar passes into the complex structure of the crowns 

 of the posterior teeth" (Huxley, Collected Papers, Vol. IV., p. 450). 



It was also advocated by Schlosser, 2 but upon different grounds, 

 namely : that in primitive jaws the upper overhangs the lower, and the 

 upper teeth fit not simply between but also slightly outside of the lower 

 ones; accordingly it is more probable that the true protocone must be 



1 " Review of the Cranial and Dental Characters of the Canidie,'' Coll tc ted Memoirs, 

 Vol. IV., p. 450. 



2 " Die Entwickelung der verschiedenen Saugethierzahnformen im Laut'e der geologisclien 

 Perioden," Sonder-Alxli: au* den Verh. d. odontoloy. Get* //*</!., lid. 3, Heft 2 u. 3, 1S91. p. ( .i 



*[But see pp. 126, 2-2-3 Addendum. ED.] 

 t [Erroneous. See p. 225. ED,] 



