1899.] 



THE CAKPUS OF CTEXOMlfS. 



429 



" Carpus of Ctenomys, etc. 



Fig. 1. Ctenomys sp. (Br. Mus. No. 97. 10. 3. 68). Left manus, palmar view. 

 R— radius; /•.■>»= racliale niargiuale. U = u]na; j9^r= proximal pisiform ; 

 pd = A\f.\a\ pisiform. — Fig. 2. Ctcnoini/s sp., Prov. Salta (Argentina). Left 

 carpus, palmar view. — Fig. 3. Same specimen as Fig. 2. Left carpus, dorsal 

 view. a = interplialangeal dorsal ossicle. — Fig. 4. Mui^ macleari Thos., jun. 

 Eiiglit carpus, palmar view. rw=proximal marginal radiale. .r=accessory 

 palmar ossicle of the carpus; sc./= " scapholunar." — Fig. 5. Arvicanthis 

 niloticus. Eight carpus, palmar view; rM.^r=proximal marginal radiale ; 

 r.7»c^=distal marginal radiale. — Fig. 6. LepidolemtirmicrodonM-dj. Right 

 carpus, palmar view. /=lunar ; M/ft = ulnare. The bones marked a; and ^ 

 (pisiform) are separated by a short meniscus of connective tissue, which 

 unfortunatelj'is made to appear in the figure as a distinct bone. — Figs. 1-3 

 and 6 slightly over nat. size ; figs. 4 and 5 about double nat. size. 



I. 



On the dorsal side of the thumb (a, fig. 3), overiapping the inter- 

 phalangeal articulation, is a lengthened ossicle, attached by a strong 

 ligament to the proximal end of the ungual phalanx, and gliding 

 on the capitulum of the second. The anterior and posterior 

 extremities of this ossicle are slight!}^ swollen, the shaft being 

 restricted, so that it somewhat resembles a diminutive phalanx. 



So-called sesamoid bones have been here and there noticed, as 

 rare occurrences, on the dorsal surface of the phalangeal articu- 

 lations in Mammalia; they will be enumerated hereafter ; bat I have 

 nowhere found a mention of an ossicle on the interphalangeal 

 articulation of the thumb. On iuA-estigating the matter more 

 closely, I have found the same ossicle constantly in the four species 

 of Mus up to the present examined, including our two common 



