| OCR Text |
Show 212 DR. C. J. FORSYTH MAJOR ON MIOCENE SQUIRRELS. [Feb. 28, opinion allows only for a movement in the longitudinal direction), this mechanical action of tbe jaw would be followed by the wearing away first of all of the middle row of the upper molars, which rubs against the inner and outer side respectively of the outer and inner row of the opposing tooth. Compared with tbe types of " Tripriodon ccelatus " 1 and " caperatus" 2, Bipriodon robustus shows its inner cusps unproportionally extending outwards, and this would justify the supposition that the remnants of two cusps of the middle row have united with what remains of the enamel-folds of the two inner cusps, a mode of coalescence which we often see realized in worn molars. I therefore fail to find in Marsh's previous publications the proofs of the statement made by him3 that one series of upper molar teeth of Cretaceous Allotheria has but two rows; although from certain analogies-with the molars of Mus on the one side, those of Cricetodon on the other-analogies which may hereafter turn out to be homologies, I am quite willing to admit that Prof. Marsh may have in his hands the means for proving it. I cannot admit Osborn to have satisfactorily shown that in this group of Cretaceous Allotheria there are lower molars with three rows of cusps. If there were three rows of cusps with two longitudinal grooves in the lower molars, we would have to urge four rows of cusps with three longitudinal grooves in the upper molars ; these have not been forthcoming up to the present date. Thus there seems to be no reason for denying Marsh's statement that "the lower molars .... although differing widely in the form and structure of their crowns, have only two parallel series of crescents or tubercles, an outer and inner row, with a groove or valley between them " 4. For m y present purpose it may be sufficient to point out, that both authors agree in stating that there are several forms with upper molars composed of three longitudinal rows of cusps with two grooves between them, to which correspond two longitudinal rows with one groove in the lower molars ; but, as stated before, they have not furnished sufficient proof for their opinion that these molars belong to an aberrant Order of Mammalia. In comparing the teeth in question with those of the Muridae and more particularly of Mus, it becomes evident that the main differences between them consist iu this, that whilst in the Cretaceous molars the prevailing division is effected by longitudinal grooves, in Mus, on the contrary, the molars are deeply divided by transverse grooves : the longitudinal grooves in this genus, of which there are two in the upper, and one in the lower molars, being relatively shallow. For further particulars on this argument, as far as it relates to Muridae, I refer the reader to a paper by Hensel on Mus orthodon from the Ossiferous Breccias 1 L. c. part I. pi. ii. figs. 19, 20. 2 L. c. part III. pi. v. fig. 2. 3 L. c. part III. p. 253. 4 Ib. p. 253. |
