312 PROCEEDINGS or THE GEOLOGICAL SOCIETY. 



We have seen that in three out of the four groups of the Probos- 

 cidea already considered, each is susceptible of being divided into 

 two subordinate series, namely, the " Eurjcoronine," in which the 

 molar-crowns are broad, the ridges transverse, and the valleys open, 

 and the " Stenocoronine," in which they are narrow, and the valleys 

 are obstructed by outlying tubercles. These two types, under pecu- 

 liar modifications, are equally present among the forms referable to 

 Euelejphas, and the distinctive marks upon which they are foimded 

 furnish excellent help in determining the distinctness of the species. 

 They are in some respects nice in degree, but at the same time, like 

 all well-founded distinctions in nature, they are very constant. In 

 order to facilitate the determination of the " ridge -formula " in the 

 fossil forms, the characters of the teeth in the existing species will 

 first be considered. But it is necessary to give some preliminary ex- 

 planations of the modifications of the dental characters in the molars 

 of the Euelephants, and of the terms that are here used to express 

 them. 



The folded crown of the molars in the groups Trilophodon, Tetra- 

 Jopliodon, and Stegodon is composed of three or more, regularly or 

 irregularly transverse, wedge-shaped cores of ivory, arising from a 

 common base, and covered by a shell of enamel, which is uniformly 

 reflected over their apices and over the reentering angles at their 

 base. These divisions are called " ridges " or " coUiculi," and the 

 interstices or valleys between them "vaUicute": though usually open 

 in the Mastodons, the latter are in the Stegodons occupied by an 

 enormous mass of cement, forming reversed wedges in relation to the 

 ivory cores. The layer of enamel thus alternates with the ivory and 

 cement, and, being of uniform thickness throughout, it is the only 

 portion of the crown-materials to which the term " plate," " lamina," 

 or " lamella " can with propriety be applied. 



In the groups Loxodon and Euelephas these ridges go on increas- 

 ing in number, without a corresponding augmentation of the length 

 of the crown, so that the penultimate true molar (or last of the in- 

 termediate series), which in the Trilophodons has only three ridges, 

 in the Indian Elephant presents five times that number, or about 

 sixteen ridges. The law of compensation (" balancement " of the 

 French, and "anamorphosis " of some German authors) comes into 

 play to make the necessary adjustments. The ridges are compressed 

 and close-packed, with an attenuation of the constituent ivory, 

 enamel, and cement-materials ; but as there is a hmit to the" 

 lateral extension of the crown, from the disturbance which would 

 be thereby involved in the general construction of the head, 

 the ridges are attenuated and elongated vertically, either with 

 no increase or in an undue proportion to the increase in the 

 width of the crowns. But these compressed ridges are still the 

 homologues of the massive divisions seen in the crowns of the 

 molars of the Mastodon, and, as such, it is but correct to retain 

 the same name for them. The obvious manner in which their 

 elongation and compression afi^ect the aspect of crown is embodied 

 in the terra " coronis lamellosa," and the diiference of degree by the 



