8 EVOLUTION OF MAMMALIAN MOLAR TEETH 



support Cope's hypothesis 1 that they represent a stage of inward 

 rotation of cusps which were at an earlier stage in the same fore and 

 aft line with the main cusp. These are, that in Phascolotherium the 

 lateral cones are seen to be slightly internal to the main cone, so that 

 their median slopes descend upon the inner face ; in Tinodon, of a 

 later geological period, this disposition is slightly more pronounced ; in 

 Mcnacodon it is still more marked, but less so than in Spalacotherium. 

 These genera, although evidently in two different lines of descent, afford 

 the desired transition stages. The Spalacotherium [Peralestes] molar as 

 seen from above ~ has a striking resemblance to the anterior sectorial 

 triangle of the Stypolophus or Didymictis molar of the Puerco. It is, 

 in fact, sub-triangular, the superior molars probably having the lateral 

 cones rotated outwards, so that the upper molars form an alternating 

 series, the ridges connecting the main and later cones acting as 

 sectorial blades." Again, in the " Evolution of Mammalian Molars to 

 and from the Tritubercular " type, Osborn pointed out (Amer. Naturalist, 

 Dec., 1888, p. 1075) that "it has been assumed by Cope and the 

 writer (op. cit., p. 243) that the para- and meta-conids were first 

 formed upon the anterior and posterior slopes of the protoconid and 

 then rotated inwards, but it is also possible that they were originally 

 formed upon the inner slopes." There is thus evidence for cusp 

 rotation, but it is not an essential part of the tritubercular theory, 

 because, as above stated, the denticles may have arisen on the inner 

 and outer sides of the cone from the outset (see p. 33). 



The third suggestion is that after the main cone had been estab- 

 lished the lateral cusps or denticles arose as dngules on the broad 

 external cingulum of the upper molars and from the broad internal 

 cingulum of the lower molars. This hypothesis, suggested by Osborn 

 (Mcsozoic Mamm., p. 245) from a study of the molar teeth of the 

 Jurassic Amblotheriidiv, has been supported by the observations of 

 Gregory. The comparison of the molar teeth of such forms as 

 Amblotherium, Phascolestes, Dryolestes (see pp. 29, 30), lends support to 

 this view, which is more fully discussed on page 33 (footnote). 



The fourth suggestion or hypothesis is the newest ; it springs from 

 embryological evidence (Woodward, Tims) and from another interpreta- 

 tion of the palseontological evidence (Gidley). It ends with the idea 

 that the oldest cone in the upper molars is the paracone (of Osborn) 

 on the outer side of the crown, from it extends inward a broad ledge 

 like a heel which finally rises up and secondarily forms the prominent 

 protocone (of Osborn). According to this hypothesis the paracone is 

 the primary (or reptilian cone), the protocone is secondary or a deriva- 



111 The Creodonta," American Naturalist, 1884, p. 259. 

 2 Owen, The Mesozoic Mammalia, Plate I., Fig. 32c. 



