| OCR Text |
Show 206 mi. c. J. F O R S Y T H M A J O R O N M I O C E N E SQUIRRELS. [Feb. 28, and the external cusps of inferior, molars have to fit into the hollows or valleys of the opposing teeth. W h e n there is only one cusp exclusively or prominently developed on the inner side of a superior molar, as in the so-called tritubercular, or at a lesser degree in the trigonodont type, this single cusp fits in the more or less cup-like depression of the posterior moiety of the corresponding 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. And, vice versa, thepostero-eattemal cusp of inferior molars, that is the hypoconid of the " heel," has to fit in the median valley of superior molars, formed essentially by the three cusps of what trituberculism 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 very rare an occurrence in Sciurinee, that up to the present day it was known only in some fossil forms (Sciuroides, Pseudosciurus, Ischyromys), whereas it is the predominant feature amongst Ungulata. To these two inner cusps correspond two hollows or valleys of the inferior tooth. The relative development 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 hypocone, a later addition according to the tritubercular theory: when the hypocone is suppressed, as in the pure tritubercular type, or feebly developed, as in the trigonodont 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, w e are able to state that the transformation, viz. the reduction and simplification, proceeds from the inner side outwards in superior, and from the outer side inwards in inferior molars. Here I have to repeat what I said with regard to semi-hypsodont teeth, viz., 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 farther stage of semi-hypsodontism, as presented by the rooted molars of Miocene Lagomorpha (Lagodus), 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 brachydont as well as a complex conformation. The fifth point advanced was stated as follows :-The now prevalent 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, |
