192 TEN years' progress IX VERTEBRATE PALEONTOLOGY 



remote predecessors even of the Mesozoic Tritubercnlata the upper 

 molars were already widened transversely as "in certain Theriodonts, and 

 that the inner part, later giving rise to a distinct protocone. fitted behind 

 the summit of the small, heelless, lower molars. But while opinions 

 differ as to the ultimate origin of the "tritubercular" t^^e or tjipes, and 

 while the homologies of the cusps may not be as universal as it was for- 

 merly believed, yet there can now be very little doubt that many or per- 

 haps all placental orders at one time passed through a stage in which 

 the upper molars were trigonal, the lower tuberculosectorial. For this 

 conclusion strong evidence was adduced by Osborn and the writer in 

 1907.^^ And the conclusion that the trigonal-tuberculosectorial types is 

 primitive is somewhat strengthened by the independent evidence that 

 most placental orders have been derived from forms having a dental 

 formula of '•,••'• ; for, wherever clear traces of this formula are ob- 

 served, then the molars of the earlier members of such groups show a 

 distinct approach either to the "sexitubercular" molars of the Cond;y- 

 larths or to the trigonal molars of the Insectivore-Creodont group ; in 

 other words, they lead back toward the trigonal-tuberculosectorial type. 

 This is notably the case with the extinct South America "Ungulates," 

 many of which retain the complete placental formula in spite of diverse 

 specialization in the form of the teeth. And although in the h^-psodont 

 forms the so-called tritubercular pattern is largely obliterated, yet in the 

 small primitive bunodont t^'pes from the Xotostylops beds the trituber- 

 cular pattern becomes very clear.-- AVe may even go further and say 

 that whenever the dentition of the early members of a race approaches 

 the normal Placental arrangement of the incisors, canines,' and simple 

 premolars, then that race is leading l)ack toward omnivorous or insect- 

 ivorous-carnivorous types with trigonal-tuberculo-sectorial molars. 



Insectivora 



In the field of recent Insectivores, Leche has continued his important 

 series of monographs, the section published in 190T-^ dealing with the 

 dentition, anatomy, and osteology of the Zalambdodonts. Weber's-* 

 treatment of the Insectivora is also noteworthy. The genetic relations 

 of the Insectivora were considered in some detail by the writer in 1910^^ 

 and some of his principal conclusions regarding the Insectivora were as 

 follows : 



^ Evolution of Mammalian Molar Teeth, pp. 91-192. 



» Gregory, op. cit., pp. 373, 376, 379. 



23 Chuns Zoologica. 4to, pp. 1-157, p. 11, i-iv. 



2* Die Saiigetiere. 



» Op. cit, pp. 231-292. 



