430 DB. C. I. PORSYTH MAJOR ON [A^pr. 18 



larj^er species ; and also in the Malagasy Eodent Brachyuromys, 

 in Spalax, and in Lar/omys. I believe it to be a frequent occurrence 

 amongst 3Iammalia, but to have escaped notice, because it is always 

 cut away in prepared skeletons. Ttie individuals in which it was 

 found have all been dissected under my 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 ) : thig 

 on the ground that the proximal of the three segments has a proxi- 

 mal epiphysis characteristic of the phalanges, but not the distal 

 one characteristic of metacarpals and metatarsals.^ 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 meta- 

 carpal or metatarsal bone, and . . . that we must distrust the position 

 of the epiphysis to these bones as the ground of a homological 

 distinction." ^ Dollo has since shown that in the young Varanus all 

 the metacarpals and metatarsals have a proximal as well as a distal 

 epiphysis ^ ; 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 meta- 

 carpals and metatarsals had originally likewise two epiphyses. 



Having discarded as invahd 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. Pfltzner 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 this ' that it is the middle 

 phalanx which has disappeared, and that its disappearance is due 



' Struthers: "On Variation in the number of Fingers and Toes, etc." 

 Edinb. New Philos. Joui-n. vol. sviii. p. Ill (1863). 



- " On the Difference in tlie Mode of Ossification of the first and other Meta- 

 carpal and Metatarsal Bones." Journ. Anat. & Phys. iii. pp. 131-146 (1869). 



3 Zool. Anz. vii. p. 80 (1884). 



* W Pfitzner, "Die kleine Zehe." Archiv f. Anat. u. Entwicklungsgesch. 

 p. 34(1890). 



' L. c. pp. 34, 35 : " Ich glaube somit annehmen zu miissen, dass auch aie 

 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 gekominen ist, dass immer das jeweilige Endglied das 

 nachstfolgende durch Verschmelzung sich assimiliert hat." — See also Pfitzner, in 

 Morph. Arb. i. p. 605 (1892) : "Das Interphalangealgelenk des ersten Fingers 

 bin ich geueigt mit dem proximalen Interphalangealgelenk der anderen Finger 

 zu homologisiren, seine Endphalanx als Verschinelzungsproduct von Mittel- 

 phalanx und urspriinglicher Endphalanx anzusehen." 



