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Show 152 PROF. G. B. HOWES AND W. RIDEWOOD ON [Mar. 6, betrachtet, werden auch furderhin die • uberzahligen' Finger Zehen .... sondern als atavistische Bildungen angesehen werden diirfen." , Baur is, of all later writers, the one who has done most to combat this doctrine. He advances (1, p. 68 et seq.) equally good arguments for regarding these so-called supernumerary digits as purely secondary and adaptive structures, laying, at the same time, great stress upon their late appearance, especially in the case of the pre-hallux itself. The advocates of the opposite belief1 seek shelter under the stronghold of the Enaliosauria, but recent investigation at least suggests that the paddles of those beasts were specialized derivatives of pentadactyle predecessors2. W e trust to have already shown satisfactorily that the naviculare can no longer be regarded as the hallux-tarsal, as Born supposes ; and that admitted, it follows that the pre-hallux conforms to the structural requirements of a sixth digit. More than this cannot be said at present, and further speculation would be useless until the connecting link between the cheiropterygium and its piscine predecessor shall have been discovered. For this we look to the palaeontologist. Setting aside further discussion as to the exact significance of the pre-hallux itself, we cannot refrain from regarding that fragmentary dismemberment of its outer free border, above represented, as an additional argument for the views of Leydig and Baur. If the converse one be justifiable, we should have ample ground for pleading the cause of an octodactyle " Urform," and this would be prima facie no advance at all. There is a growing tendency to attach too much importance to gristly fragments such as those now under consideration, and it is binding on those who may yet deal with these supposed vestiges (especially as manifested in the higher Vertebrata), that they shall determine at the outset, with greater accuracy than has hitherto been done, what precise relationships they bear to the soft parts. If not, the question bids fair to be reduced ere long to the condition of a reductio ad absurdum. FORE FOOT. a. Metacarpals and Phalanges.-The pollex of the Anura under- 1 Parker writes (P. E. S. vol. 42 (1887), p. 57), " I have frequently noticed that aborted parts, like overshadowed plants, are late to appear, and soon wither, or are arrested in their growth." a Baur, " Bemcrkungen iiber Sauropterygia und Ichthyopterygia," Zool. Anzeiger, 1886, pp. 245-252; also " Die Abstammung d. Amnioten Wirbelth.," Biolog. Centralbl. Bd. 7 (1887), pp. 481-493. See also " O n the Phylogenetic Arrangement of the Sauropsida," ' Journal of Morphology,' Boston, vol. i. no. 1 (1887), pp. 93-104 ; the discovery by Gadow and Baur (herein recorded) of supernumerary phalanges in the manus of Halicorc and Manatus is most welcome at this juncture. See also Baur, " "Ueber den TJrsprung d. Extremi-taten der Ichthyopterygia," Bericht u. d. xx. Versammlung des Oberrheinischen Geolog. Vereins, 1887. |
