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Show 434 DR. C. I. FORSYTH MAJOR ON [Apr. 18, Neither is the large cartilage supporting the patagium of Sciuro-pterini an " ulnare antebrachii," as supposed by Thilenius ', from an erroneous interpretation of the figured skeleton of " Pteromys volucella."2 In the only skeleton of a Flying-Squirrel in the Nat. Hist. Mus. in which this part has been preserved (Pteromys magni-ficus), it is chiefly attached to the distal end of the pisiform and, besides, by a much smaller ramification, to the tuberosity of tbe fifth metacarpal. To judge from its position, it is therefore in the main the homologue of the distal pisiform of Muridae and Ctenomys, and possibly of the pisiform epiphysis of many other Mammals. A dependency of the pisiform is likewise the curious sub-cylindrical structure which in Chrysochloris " simulates a third antebrachial bone," and is by Dobson 3 and others taken for the ossified tendon of the flexor digitorum profundus. In fact, the tendons for the four digits take their origin from the distal end of this bone ; from this it does not, however, necessarily follow that the bone is an ossified tendon. At the dorsal side of its distal base it is provided with two facets, the larger ulnad one for the " ulnar sesamoid," the smaller radiad one for a volar and distal projection of the lunar. More about this remarkable structure will be said elsewhere. III. In the fore-limb of Ctenomys occurs further an unusually prominent process of the radius, on the volar side of its distal ulnad end (figs. 1 & 2). In order to come to a clearer understanding, I looked for younger stages of Ctenomys. None being available, I resorted to Mus, in younger specimens of which I find in the same place, intercalated between the pisiform and the radius, a distinct ossicle (x, fig. 4), which later on becomes fused with the radius, thus forming the above-mentioned process. I have since found the same ossicle, though much smaller, in the fore-limb of a young individual of the Malagasy Rodent Brachyuromys rami-rohitra, as well as in Arvicanthis (x, fig. 5). In the Rodents in which the ossicle occurs, no distinct lunar is known ; they are therefore said to have a scapho-lunar bone, it being supposed that the lunar is fused with the scaphoideum. At one time a similar statement was made with regard to Marsupials, but eventually in several genera a distinct lunar bone, although sometimes very minute, has been traced. In Phascolarctus no distinct lunar is known in the adult: however, in his recent memoir " Beitrage zur Entwicklungsgeschichte und Morphologie des Hand- und Fuss-skelets der Marsupialier," 4 Emery has described and figured sections of embryonic stages of Phascolarctus cinereus, in which appears an element which " on account of its 1 Morph. Arb. v. p. 508 (1896). 2 Owen, ' Anatomy of Vertebrates,' ii. p. 384, fig. 247 a (1866). 3 G. E. Dobson, ' Monograph of the Insectivora,' p. 121 (1882). 4 Semon's ' Forschungsreisen, II.' v. pp. 372, 373 (1897). |