20(5 DR. U. J. iUilSYTH MAJOll 0^' MIOCENE SQUIEiRELS. [l^eb. 26, 



aud the external cusps of iuferior, molars have to fit iuto the 

 hollows or valleys of the opposing teeth. When there is only one 

 cusp exclusiA'ely or prominently developed on the inner side of a 

 superior molar, as in the so-called tritubercular, or at a lesser 

 degree in the Irigonodont type, this single cusp fits in the more 

 or less cup-like depression of the posterior moiety of the corre- 

 sponding inferior molar ; in other words, the protocone, — viz., the 

 oldest element of upper molars, according to the tritubercular 

 theory — fits in what the theorists consider to be the most modern 

 part of inferior molars. Aud, vice versa, ilwpostero-e.vtevnal cusp of 

 inferior molars, that is the Ju/j'oconid oi the " heel,*' has to fit in the 

 median valley of superior molars, formed essentially by the three 

 cusps of M hat trituberculism considers to be the prinntive trigon 

 of upper molars. 



The special development of two inner cusps in superior molars 

 has been shown to be so xevy rare an occurrence in Sciurino:, that 

 u]) to the present day it \\ as known only in some fossil forms (Sciu- 

 roldcs, P.<ei(doscim'us, Isch>/rotni/s), whereas it is the predominant 

 feature amongst Ungulata. To these two inner cusps correspond 

 two hollows or A'alleys of the inferior tooth. The relative de^elop- 

 ment of the anterior valley, formed by the so-called primitive trigon 

 of inferior molars, corresponds to that of the postero-internal 

 cusp of superior molars, — the luipocone, a later addition according to 

 the tritubercular theory : when the hypocoue is suppressed, as in 

 the pure tritubercular type, or feebly developed, as in the trigou- 

 odont type, the anterior valley in the lower molars is atro])hied or 

 insignificant. It is well deA'eloped, to receive the hypocone as a 

 pestle, when this last has the same or almost the same size as the 

 protocone. 



Fourthly, we are able to state that the transformation, viz. the 

 reduction and simplijication, proceeds from the inner side outivards 

 in superior, and, from the outer side inwards in inferior molars. 

 Here I have to repeat what 1 said with regard to semi-hypsodont 

 teeth, \VA., that the vertical elevation of the crown, the first stage 

 towards hypsodonty, aluays has its starting-point from the inner 

 side of upper molars. A farther stage of semi-hypsodontism, as 

 presented by the rooted molars of Miocene Lagomorpha {Lagodus), 

 shoMs how hypsodontism gradually extends towards the outer 

 side, accompanied by a gradual and essential change of the pattern 

 of the crown. In the same way the inner root, \Ahich ultimately 

 will remain open, gradually extends outw^ards, increasing in size, 

 and receives a coating of enamel. This process is so slow, that for 

 a long time the outer side retains a brachydont as well as a 

 complex conformation. 



The fifth point advanced was stated as follows : — The now pyre- 

 valent transverse arrangement of cusps or lobes is not the primitive 

 condition, but a specialized pattern of the crown. What, then, was 

 the primitive condition ? The primitive generalized pattern was 

 the exact reverse, viz., the arrangement of cusps in longitudinal 

 series, separated by longitudinal grooves or valleys. This is, indeed, 



