CHAP. Vl] THE DENTAL SYSTEM OF THE PRIMATES 295 



sequence 1 . Antero-posterior concrescence is the keystone of Tims' 

 theory as regards the molar teeth, and it must be admitted as 

 unfortunate that the evidence of concrescence is not yet con- 

 vincing, except in Sphenodon (a reptile), in Ceratodus (a Dipnoan 

 fish) 2 , and perhaps in the Wallaby (a Marsupial), as suggested 

 lately (1911) by Tims and Hopewell Smith. 



Concrescence indeed has been assigned, as a name, to a definite 

 theory, which would seem (cf. Marett Tims, J. A. P., Vol. xxxvu. 

 p. 134), to have been first advanced by Gervais. Its chief advocates 

 in later years have been Rose and Ktikenthal, against whom we 

 may set Leche (Bibl. Zool. Heft 17, pp. 154, 155). The theory 

 postulates the possibility of primitive conical dental elements 

 fusing to form complex teeth : and the elements which thus fuse 

 may belong to one and the same, or to different dentitions. 



Rose {Anat. Anz. VII. p. 395) declares himself an advocate 

 of the Concrescence theory as accountable for the origin both of 

 premolar and molar teeth, and has represented his views in an 

 elaborate diagram. It may be noted in conclusion of this brief 

 notice, that Rose postulates the original presence of from 46 — 52 

 single conical teeth, which by fusion have constituted the dentition 

 actually found in the Hominidae. Such a number of conical 

 teeth is submitted to be not by any means unusual in the 

 dentition of reptiles. 



D. The Plexodont theory of Ameghino can only be men- 

 tioned here. It postulates the concrescence of primitive conical 

 teeth in groups of four, with the production of compound four- 

 cusped teeth. Further details are given by Osborn (op. tit. 1907, 

 pp. 201 et seq.). 



The foregoing notes contain an attempt to set forth in the 

 most concise manner the several views which have been taken, 

 and the hypotheses which have been advanced in explanation, 

 of what is undoubtedly a difficult problem, and one upon which 

 much light remains to be shed. 



1 In the Primates and most Eutheria, concrescence of pairs of such elements 

 is suggested, while in the Elephants a greater number of elements would appear 

 to have been fused together. 



2 But not in Protopterus, another Dipnoan fish (cf. Rose, Anat. Anz. vn. 

 p. 399). 



