206 JJll. C. J. iuESlTH MAJOE OK MIOCENE SQUIERELS. [Feb. 28, 



aud the external cusps of inferior, molars have to fit into the 

 hollows or Talleys of the opposing teeth. When tliere is only one 

 cusp exclusively or prominently developed on the inner side of a 

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

 degree in tlie trigonodont type, this single cusp lits 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 trituhercular 

 theorj- — fits in what the theorists consider to be the most modern 

 part of inferior molars. Aud, vice versa, Wxe postero-eA-tevnal cusp of 

 inferior molars, that is the hijpoconid oi the "heel," has to fit in the 

 median valley of superior molars, formed essentially by the three 

 cusps of uhat tritubei'culism considers to be the primitive trigon 

 of upper molars. 



The special development of two inner cusps in superior molars 

 has been shown to be so \'ery rare an occurrence in Sdnrince, that 

 uj) to the present day it ^\•as known only in some fossil forms (Sciu- 

 roides, Pseudosnarus, Ischi/romifs), whereas it is the predominant 

 feature amongst Ungulata. To these t\^o inner cusps correspond 

 two hollows or valleys of the inferior tooth. The relative develop- 

 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 hi/jjocoxe, a later addition according to 

 the trituhercular theory : when the hypocone is suppressed, as in 

 the pure trituhercular type, or feebly developed, as in the trigon- 

 odont type, the anterior valley in the lower molars is atrophied or 

 insignificant. It is well developed, 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 simjTliJi cation, j^irocecds from the inner side outwards 

 in sv^H'rior, and from the outer side inwards in inferior molars. 

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

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

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

 side of upper molars. A fai'ther stage of semi-hypsodontism, as 

 presented by the rooted molars of Miocene Lagomorjiha (Larjodus), 

 shows 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, which ultimately 

 will remain open, gradually extends outwards, increasing in size, 

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

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

 complex conformation. 



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

 vcdent traiisverse arrangement of cusps or lobes is not the primitive 

 condition, hut 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, 



