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Show 430 DR. C. I. FORSYTH MAJOR ON [Apr. 18 larger species ; and also in the Malagasy Bodent Brachyuromys, in Spalax, and in Lagomys. I believe it to be a frequent occurrence amongst Mammalia, but to have escaped notice, because it is always cut away in prepared skeletons. The individuals in which it was found have all been dissected under m y supervision. In Mus and Lagomys the ossicle is of a more irregular shape and reduced in size. The suggestion which at once offers itself is, that we have before us the missing skeletal element of the thumb, which has become reduced after having been displaced from its original position, and is now gradually vanishing. In the following I shall consider the greater or less probability of such a hypothesis. It has been maintained at one time, that the thumb and the toe have the same number of three phalanges as the other fingers and toes, and that the missing bone is a metacarpal (metatarsal): this on the ground that the proximal of the three segments has a proximal epiphysis characteristic of the phalanges, but not the distal one characteristic of metacarpals and metatarsals.1 Allen Thompson pointed out, in an interesting article, that the above is by no means the rule; his observations led him to the conclusion " of the inconstancy of the absence of a distal epiphysis in the first metacarpal or metatarsal bone, and... that we must distrust the position of the epiphysis to these bones as the ground of a homological distinction." 2 Dollo has since shown that in the young Varanus all the metacarpals and metatarsals have a proximal as well as a distal epiphysis 3; a fact which, held together with the cases in Mammalia quoted and described by A. Thompson, and to which I could add further instances, makes it probable that all the Mammalian metacarpals and metatarsals had originally likewise two epiphyses. Having discarded as invalid the reasons which would assign three phalanges to the first digit and toe, the next question to answer is, whether the missing phalanx is the first, the second, or the third. Pfitzner has pointed out that, in those Mammals in which the ungual phalanx has either totally (some Monkeys) or almost totally (Wombat, Elephant) disappeared, the next phalanx shows not the least tendency to assume the form of the former *. He concludes from this5 that it is the middle phalanx which has disappeared, and that its disappearance is due 1 Struthers: "On Variation in the number of Fingers and Toes, etc." Edinb. New Philos. Journ. vol. xviii. p. Ill (1863). 2 " On the Difference in the Mode of Ossification of the first and other Metacarpal and Metatarsal Bones." Journ. Anat. & Phys. iii. pp. 131-146 (1869). 3 Zool. Anz. vii. p. 80 (1884). 4 W . Pfitzner, " Die kleine Zehe." Archiv f. Anat. u. Entwieklungsgesch. p. 34 (1890). 5 L. c. pp. 34, 35: " Ich glaube sorait annehmen zu miissen, dass auch die Zweigliedrigkeit des Daumens und der grossen Zehe der Saugethiere und des Menschen, und ebenso die Dreigliedrigkeit der iibrigen Zehen und Finger in der Weise zu Stande gekommen ist, dass immer das jeweilige Endglied das nachstfolgencle durch Verschmelzung sich assimiliert hat."-See also Pfitzner, in Morph. Arb. i. p. 605 (1892): "Das Interphalangealgelenk des ersten Fingers bin ich geneigt mit dem proximalen Interphalangealgelenk der anderen Finger zu homologisiren, seine Endphalanx als Verschmelzungsproduct von Mittel-phalanx und urspriinglicher Endphalanx anzusehen." |