434 DB. C. I. FOBSTTH MAJOB ON [^P^- 18, 



Neither is the large cartilage supporting the pataglum of Sciuro- 

 pterini an " ulnars antebrachii," as supposed by Thilenius \ from 

 an erroneous interpretation of the figured skeleton of " Pteromys 

 volucella"^ In the only skeleton of a Flying-Squirrel in the Nat. 

 Hist. Mus. in which this part has been preserved (Pteromys magni- 

 Jicus), it is chiefly attached to the distal end of the pisiform and, 

 besides, by a much smaller ramification, to the tuberosity of the 

 fifth metacarpal. To judge from its position, it is therefore in the 

 main the homologue of the distal pisiform of Muridse 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 ^ 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 structvire 

 will be said elsewhere. 



III. 



In the fore-limb of Ctenomys occurs further an unusually pro- 

 minent 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 Eodents in 

 M'hich 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 oue 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," * Emery has de- 

 scribed 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). 



■* G. E. Dobson, ' Monograph of the Insectirora,' p. 121 (1882). 

 * Semon's ' Forschungsreiseu, II.' v. pp. 372, 373 (1897). 



