CHAP. Vl] THE DENTAL SYSTEM OF THE PRIMATES 251 



relations in point of size : the last is often the smallest of the three ; in form 

 the crowns are less elongated in the antero-posterior direction than are the 

 corresponding teeth in Gorilla or Anthropopithecus. The cusps are commonly 

 four in number, and they are distinct, separated by grooves forming a crucial 

 fissure. But in many cases, and particularly among the primitive and pro- 

 gnathous coloured races, a fifth cusp (the "hypoconulid" of Osborn, Evolution 

 of Mammalian Molar Teeth, 1907, p. 81) is found usually on the posterior 

 margin and between the two normal cusps. 



The comparison of the length of the post-canine series of teeth has been 

 studied by Flower (cf. the Journal of the Anthrop. Institute, 1885), who devised 

 an index for comparative purposes : this dental index is based on the relation 

 of the dental length (that is the combined lengths of the crowns of the 

 post-canine teeth) to the length of the cranio-facial axis. (Cf. Fig. 150 B, N.) 



The index is arrived at from the formula : 



-r. , . . , length of the premolar and molar crowns x 100 



Dental index = — — =— —. ^ j-t-i —. — — , 



basi-nasal cranial length 



and crania are classified according to the numerical value of that index. 

 Thus the limits of the class divisions are at 42 and 44. Crania with dental 

 indices of a greater numerical value than 44 are distinguished as megadont, 

 and such crania are found to prevail in the Oceanic-negro races, while when 

 a comparison is made with the Simiidae, these too (with the exception of 

 Hylobates, and perhaps only in individual examples of these) are very dis- 

 tinctly megadont, the average value of the index far exceeding 44 and rising 

 to 54 or more. (For other data cf. Chapter xiv.) 



B. THE INCISOR TEETH. 



The formula throughout the Primates is i, § ; they are borne by the 

 premaxilla in the upper jaw. 



LEMUROIDEA. Lemuridae. Lemur varius. Fig. 180. 



The characteristic features of the incisor teeth of the Lemuridae are as 

 follows : (a) the upper teeth are extremely small (in some, the extinct Lemur, 

 e.g. Megaladapis madagascarensis, they are vestigial or absent, cf. Standing, 

 Zool. Trans., May, 1908), and the median teeth are separated by a wide gap 

 in the middle line of the face; (b) the lower teeth are larger and curiously 

 styloid, they are also procumbent, that is, they are directed almost horizontally 

 forward from the mandible and cannot therefore be of much use as incisor 

 teeth in the ordinary acceptation of the term ; (c) the apparent occurrence of 

 three incisor teeth on each side in the mandible is explained by the view 

 which regards the lower canine teeth as having assumed the morphological 

 characters and the position of lower incisor teeth (it will be remembered, 

 v. ante, p. 242, that the succeeding tooth, the first premolar, has to some 

 extent replaced the normal lower canine teeth). 



