262 PKOCEEDIKGS OP THE GEOLOGICAL SOCIETY. 



part of this paper, the higher the numerical expression of the " ridge- 

 formula," in the species, the more liable is the number ef ridges to 

 vary within certain limits dependent on the race, sex, and size of 

 the individual, and the molars of the lower jaw often exhibit an ex- 

 cess. After examining a very lai'ge number of specimens of all ages, 

 the prevailiug numerical expression of the ridge-formula, exclusive 

 of " talons," in E. (Stef/odon) insignis has appeared to me to be 

 thus : — 



Milk-molars. - True molars. 



2 + 5 + 7: 7+ 8 +10-11 



2 + 5 + 7: 7+(8-9) + ll-13' 



I have abeady remarked that all the known species of Stecjodon 

 belong to that species of the Proboscideans in which the ridges are 

 transverse, and the valleys open. It may be expected, without 

 much temerity, that other species remain to be discovered in the 

 fossil state, in which the mammillae will be disposed more or less 

 alternately, with outlying tubercles and interrupted valleys, as in 

 the " Stenocoronine " type. 



lY. PENTALOPHODOISr. 



From the circumstance that so many Mastodons present the 

 ciphers either 3 or 4 constantly in the ridges of the intermediate 

 molars of two groups of species, and that in the next allied 

 group, Stegodon, EJephas {Stegodoii) Cliftii in like manner presents 

 the cipher 6 in two of the same teeth, while the prevailing number 

 augments in ^. (Stegodon) bombifrons and E. (Stegodon) insignis, with 

 faith in the harmony of nature it might have been with some con- 

 fidence anticipated that another Proboscidean type remained to be 

 dicovered in the fossil state, intermediate between Tetraloplwdon 

 and Stegodon, in which a quinary ridge-formula would be presented, 

 constituting a third subdivision of the genus Mastodon, to which 

 the name of Pentcdopliodon would be applicable. 



It appears to me that the Indian fossil species M. (TetralopJiodon) 

 Sivalensis, figured in the 'Pauna Antiqua'*, presents the first indi- 

 cation in that direction. In the " intermediate molars" of this form, 

 both upper and lower, besides the usual anterior " talon " and four 

 laipge ridges, there is a fifth ridge, somewhat reduced in size, but 

 exactly corresponding with the other in form, composed of several 

 large mammillary tubercles, separated from the next ridge by a 

 valley, and throwing off' an outlying tubercle, which reduces the 

 valley, as in M. (Tetrahphodon) Arvernensis, to lateral gorges. This 

 fifth ridge is not a mere off'set from, or subordinate appendage to, 

 the fourth ridge after the ordinary manner of a " talon." It is 

 supported directly by the last fang, and is separated, both on the 

 outer and inner sides, from the latter by the intervening valley. In 

 most of the species of Mastodon having alternate mammillae, the 

 hind " talon" in the upper molars (and conversely in the lower) 

 forms a crenulated " bourrelet," which is given off" from the inner 

 posterior mammilla, descending obliquely around the base of the 

 * Op. cit. pi. 36. figs. 1-6, 



