GENERIC DISTINCTIONS OF PROBOSCIDEA. 9 



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 Mastodon, of which only a single form is at present 

 known, i» which the crowns of the ' intermediate molars ' 

 are divided upon a quinary ridge-formula. This group in 

 our arrangement would be characterized, in harmony with 

 the others, as Pentalophodon ; and it may with some confi- 

 dence be predicated that, when the dentition shall have been 

 well determined, 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 circumstance 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 

 collectively designated Mastodon Elephanto'ides by Cliffc, 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 Mastodons, 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 inter- 

 spaces 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. l It is limited to extinct forms 

 confined at present to the Indian Tertiaries. The Stegodons 

 constitute the intermediate group of the Proboscidea from 

 which the other species diverge through their dental cha- 

 racters, 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 African Elephant, the ridge-formula is also hypisomerous, 

 as in the Stegodons, the ciphers ranging from 7 to 9 in the 



1 From ariyq tectum, and oSous dens, having reference to the gable-end form 

 of the section of the ridges. 



