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Show 184 DR. 0. J. FORSYTH MAJOR ON MIOCENE SQUIRRELS. [Feb. 28, 3. To the Xerus-type belong most of the semi-hypsodont Sciurine teeth of the groups already mentioned. It is distinguished from the Sciurus-vulgaris-type by a more complete lophodontism, the crests running in a transverse direction, and the valleys being reduced to narrow but deep fissures. The main difference from Ungulate lophodont molars consists in their having not two, but four or even five crests, the anterior and posterior margin of the molar being as much or almost as much raised as the two medial crests. So that the Xm«*-type presents in upper as well as in lower molars three transverse depressions or fissurelike valleys (and four in superior molars where there are five crests). This type forms on the whole a very striking approach to the hystricomorphan molar. Twenty years ago I pointed out that two African Squirrels, Sc, congicus and Se. leucostigma (this last being now considered as a variety of Sc. pyrropus), approach the Hystrioomorpha in a further development of the characters in which the Ethiopian Xerus departs from the Sciurus-vulgaris-type. Again, the Eocene Sciuroides was shown to resemble in its molars those of the Hystricomorpha Cercolabes and Erethizon, whilst, on the other hand, the Eocene hystricomorphan Trechomys was shown to approach Sciuroides 1. Twelve years later, Schlosser, working on fossil forms, with much richer material at his command, confirmed these observations, showing that a group of Tertiary Rodentia showed characters intermediate between the Sciuromorpha and Hystricomorpha2. It is of no little interest that actually living Sciuromorpha show in their dentition, as well as in the characters of their skull, that the distinction between Sciuromorpha and Hystricomorpha is less sharp than is generally admitted. Schlosser seems not to have examined any recent Ethiopian Squirrels, as he does not mention the important characters they present. Nor has any other author, with the exception of Alph. Milne-Edwards, who incidentally mentions Sciurus pyrropus, F. Cuv., saying that "les molaires offrent une apparence toute particuliere, due a l'existence de replis d'c'mail qui s'enfoncent profondement dans la dentine, constituant ainsi de veiitables rubans. Je ne connais que cette espece sur laquelle on observe ces particularity ; cependant, chez certains Xerus, on en voit des indications "s. The fact is that the majority of Ethiopian Squirrels depart from the Sciurine type both in their superior and inferior molars, approaching towards hystricine lophodonty by the uniting of their cusps in a transverse direction, so that the characteristic cup-like ' 0. J. Forsyth Major, " Nageriiberreste aus Bohnerzen Siiddeutschlands unci der Schweiz. Nebst Beitragen zu einer vergleichenden Odontographie von Ungulaten und Unguiculaten," Palseontographica, Bd. xxii. 1873, pp. 75-130. 2 Max Schlosser, " Die Nager des Europaischen Tertiars etc.," Paheonto-graphica, xxxi. 1885, pp. 19-102. 3 H. et Alph. Milne-Edvrnrcls, " Recherehes pour servira, 1'HMoire Natitrelle des Mammiferes, ete." (Paris, 1808-1874), p. 100, |
