314 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [April 8, 



of the croTVTi of the tooth which is immediately in front of, and of 

 that which is immediately behind, the three intermediate molars : the 

 former sho\\ing invariably one ridge less, and the latter one ridge 

 more ; that is to say, the penultimate or second milk-molar in all the 

 species of Trilophodon is invariably two-ridged, and the last true molar 

 four-ridged ; while in Tetralophodon, in like manner, the former is 

 three-ridged, and the latter five-ridged, — making due allowance in 

 the last true molar for the amount of iudiridual variety presented by 

 the greater or less development of the well-known talon-complication, 

 and for its being usually more complex in the lower jaw. The "ridge- 

 formula" thus determines, with precision, five out of the series of six 

 molars developed in horizontal succession in all the true Mastodons. 



For reasons which will be explained in the sequel, it would seem 

 that there has existed in nature another subgeneric group of Mas- 

 todon, of which only a single form is at present known, in which the 

 crowns of the " intermediate molars " are divided upon a quinary 

 ridge-formula. This group in our arrangement would be charac- 

 terized, in harmony with the others, as Pentalophodon : and it may 

 with some confidence be predicated, that, when the dentition shall 

 have been well deteimined, the second milk-molar will present four 

 ridges, and the last true molar six ridges in the upper jaw. 



The Elephants, on the other hand, are distinguished from the 

 Mastodons by the absence of an isomerous ridge-formula to the three 

 intermediate molars of the upper and lower jaws ; and by the cir- 

 cumstance that the ridges, instead of being limited to three or four, 

 range from six up to an indefinite number in these teeth, in the different 

 groups of species. We have found that the numerous forms, fossil and 

 recent, may be conveniently arranged in three natural subgeneric 

 groups, founded upon the ridge-formula, in conjunction with certain 

 other dental characters. 



In the first of these groups, corresponding with the forms collect- 

 ively designated Mastodon elephantoides by Clift, the ridge-formula 

 may be said to be hypisomerous, as the difference between the crowns 

 of any two of the consecutive intermediate teeth does not exceed 

 more than one ridge, and the ciphers range in the different species 

 from 6 to 8. The ridges are not more elevated than in the true Mas- 

 todons, so that, when the teeth are sawn through longitudinally, the 

 section yields a succession of salient and re-entering angles, the height 

 of the chevron-shaped ridges not much exceeding the width of their 

 base. The enamel is very thick, and the coronal interspaces in most 

 of the species are filled up with an enormous quantity of cement. To 

 this group we have assigned the subgeneric name of Stegodon * . It 

 is limited to extinct forms confined at present to the Indian Tertia- 

 ries. The Stegodons constitute the intermediate group of the Pro- 

 boscidea from which the other species diverge through their dental 

 characters, on the one side into the Mastodons, and on the other into 

 the typical Elephants. 



In the second group, which includes the species allied to the 



* From oreyj; tectum, and 6?oi»s dens, having reference to the gable-end form 

 of the section of the ridges. 



