94 EVOLUTION OF MAMMALIAN MOLAR TEETH 



2. THE TPJASSIC MAMMALS. 



The supposed Triassic mammals again include the two grand divisions 

 of tuberculate or grinding teeth, and of pointed or piercing teeth. They 

 are distinguished as mammalian by the more or less complete division 

 of the root into two fangs. 



Among the former is the genus Microlestes of the Upper Triassic 

 or Rhsetic of Germany, known from a single molar (Fig. 48, No. 1). 

 The so-called Microlestes teeth in the Rhretic of England (Fig. 48, 

 Nos. 2, 3) are rounded or basin-shaped, with irregular tubercules on the 

 sides ; they undoubtedly belong to a different species or even genus, as 

 they are broader and more basin-shaped, resembling in fact the hinder 

 molars of Plagiaulax. 



The other division is believed to be represented by the Protodouta, 

 which we have already described in detail (pp. 18-21), with large single 

 cones, more or less regularly developed lateral denticles, and partially 

 divided fangs. Numerically, these molars have one fang and three cusps, 

 namely, protocone, paracone, metacone. 



3. MAMMALS OF THE JURASSIC. 



We now meet with the multituberculates, considerably specialized 

 into different types. 



The other mammals present two great types of teeth : first, the 

 triconodont with large main cones and lateral denticles both in the upper 

 and lower molars, as exhibited in Ampliilcstcs (Fig. 5) and Pliascolotlicrinm. 

 (Fig. 6) ; these animals are believed to be Marsupials. The second 

 type is the tritubercular, or more strictly, tuberculosectorial, that is, 

 with tubercular or crushing heel and sectorial trigon, as exhibited in 

 Amphitherium (Figs. 15, 17). (See pp. 21-30.) 



The Upper Jurassic* exhibits the surviving triconodont types, among 

 which are such teeth as those of Spalacothcrium (Fig. 11), which appear 

 to exhibit a transition between the triconodont and tritubercular. In the 

 same geological period the tritubercular types are diversified into two 

 distinct kinds, both of which have their parallels among later Tnsec- 

 tivora.t Numerically, these upper molars have three fangs and three 

 main cusps, namely, protocone, paracone, metacone, also a parastyle, 

 metaconule ; the lower molars have two fangs and from three to 

 four cusps, namely, protoconid, paraconid, metaconid and hypoconid. 



* [As stated on p. 22, the Purbeck and Atlantosaurus Beds are by some regarded as of 

 Lower Cretaceous age. ED.] 



t[7. e. one type, represented by Peramus, Amphitherium, resembles the teeth of Microp- 

 ternodus (Fig. 71) : a second type represented by Stylodon parallels the high piercing type 

 of Chrysochloris. ED. ] 



