38 
called protocone has been developed subsequently ,,as an internal 
shelf acting as a mortar for the cusps of the lower teeth"; with 
the elongation of the teeth the hypocone arose behind it. As 
regards the further development of the ""tritubercular” tooth thus 
formed, we can accept the Cope-Osborn views. He interprets the 
teeth of Centetes and Ericulus usually looked upon as typically tritu- 
berculate in another way, maintaining that the main inner cone is 
not homologous with the protocone, but with the paracone (the 
«protocone” is wanting, in Chrysochloris it is still visible as a 
little "internal shelf”); the two outer cusps are not para- and 
metacone, but para- and mesostyle. 
H. F. Osborn's "Trituberculy” (1897) is essentially a revision 
of his earlier theory with appreciation of Scott's modifications with 
respect to the upper teeth; in opposition to Forsyth Major he 
endeavours to derive.the rodents and probably also the Multituber- 
culates from trituberculate forms. 
As an advocate for the Concrescence Theory Fl. Ameghino 
repeatedly appears, especially in "The primitive Type of Plexodont 
Molars of Mammalia” 1899. In this paper he provisionally dis- 
cusses the lower molars only and finds in them as early as in the 
Cretaceous Proteodidelphys all the elements characterising the com- 
plete tuberculo-sectorial tooth; this he regards as the starting point 
for all the different tooth-types, not excluding the simpler ones 
(except Monotremata, Edentata and Cetacea), which consequently are 
not primitive, but reduced. The teeth of the Multituberculata have 
originated through an increase in the number of cusps in the 
paucituberculate teeth, from which all the other mammalian orders 
can also be derived. (Ameghino derives most of recent mammalian 
orders from Cretaceous or Eocene forms found in Argentina). And 
bis "plexodont” ancestral type must have been formed by fusion 
of several teeth: ,,Plexodonty presents itself is a primitive cha- 
racter, having made its appearence suddenly; and it is only the 
theory of fusion which can explain it in a satisfactory manner." 
(pag. 571). 
