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Show 1896.] MYOLOGY OF RODENTS. 173 In no case could I satisfy myself of the existence of an adductor or opponens pollicis. Muscles of the Little Finger.-These are not so easily made out as in the other Rodents. The abductor is present, but is not double. The flexor brevis is represented by the ulnar slip of the interosseus muscle to the little finger, when there happens to be a muscle in that position. I have never been able to find an opponens, although Windle describes it in Hydromys. Interossei.-In all the animals examined, except Georychus and Bathyergus, there were eight interossei, the inner of which formed the flexor brevis minimi digiti. In these two animals there were no interossei attached to the fifth finger, although the two sesamoid bones supposed to be developed in them were present. Practically the same arrangement was found in the Beaver. In Cricetomys, Microtus, and possibly in Cricetus, there was an adductor minimi digiti, which recalls the figure already published l of the same muscle in Ccelogenys. In Rhizomys and Gerbillus there is an adductor indicis. These two muscles are situated on a plane superficial to that of the interossei, and I have not come across any Myomorphine animal which possesses both of them, although they frequently co-exist in the Hystricomorpha. Muscles of the Trunk. Panniculus Carnosus.-In the majority of the Myomorpha the panniculus corresponds to the rodent type already described. The sterno-facialis is always present, but seldom large. In Georychus and Bathyergus the panniculus, especially the anterior part, is very well developed; iu the former the platysma is very strong and rises from the angle of the mouth and from the median raphe running back from the symphysis menti, it runs backward and upward to be lost over the region of the shoulders, though some of the more posterior fibres are attached to the metacromial process. W h e n this is removed, the whole length of the sterno-facialis and epitrochleo-facialis comes into view, the former rising from the posterior half of the sternum, the latter from the internal condyle of the humerus; they both run forward to be inserted into the fascia on the surface of the masseter. In Bathyergus the sterno-facialis has undergone greater development, and is continuous posteriorly with the panniculus of the abdomen; this I have figured, and have pointed out2 that it is a possible foreshadowing of the human sternalis muscle. The posterior part of the panniculus in Bathyergus is also well marked, and gets an attachment to the external tuberosity of the tibia and to the ramus of the ischium. In Heteromys among the Geomyidae, the pouch causes a good deal of modification in the facial panniculus. The superficial part or platysma rises from the ' P. Z. S. 1894, p. 273. 1 Journ. Auat. vol. xxix. p. xii. |