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Show 1896.J MYOLOGY OF RODENTS. 185 4. In B. and G. the sterno-scapularis is present. In R. it is absent. 5. In B. and G. the teres major is inserted in front of the latissimus dorsi, in R. behind it. 6. The coraco-brachialis is absent in 67., small in m y specimen of B., absent in Milne-Edwards's specimen. In R. the second and third parts are well marked. 7. In B. and 67. the extensor communis digitorum sends no slip to the fifth digit. In R. a slip to this digit is present. 8. There is no sesamoid bone in the tendon of the supinator brevis in B. and 67. There is one in R. 9. The pair of interosseous muscles which should be inserted into the two sesamoid bones of the fifth digit of the hand are absent in B. and 67. They are present in R. 10. The rectus abdominis does not decussate with its fellow of the opposite side in B. or 67., though it does so in R. 11. The gracilis is a single muscle in R. It is distinctly double in B. and 67. 12. The flexor tibialis joins the flexor fibularis in the sole of R. The two tendons are separate in B. and G. 13. The adductor indicis pedis is absent in 67. and B., present in R. It will thus be seen that, though there are six more or less unimportant points of resemblance between Rhizomys on the one hand and Bathyergus and Georychus on the other, there are 13 points of difference, some of which, such as nos. 3, 4, 10, 12, and 13, I regard as of great importance. The study of these marked muscular differences in animals whose habits are so much alike, and whose external appearances are so similar, seems to point to one of two conclusions. Either the external appearances are acquired by the animals living under similar conditions while the muscles tell the true tale of their different ancestry, or else the differences in the muscles are of no value for classificatory purposes. Against the latter conclusion the evidence of the myology of Bathyergus and Georychus tells strongly ; these animals are so alike in their habits, in their osteology, and in their visceral anatomy, that no one doubts that they are closely related ; they are also alike in their myology with one or two trifling exceptions. This, however, is only one instance of the close resemblance of the musculature in animals which are for other reasons regarded as akin ; and I cannot help thinking that when several important differences occur in the muscles of two animals which otherwise seem closely related, the muscles are trustworthy guides, because, taken as a whole, they are less likely to adapt themselves quickly to changed conditions than are other structures. With regard to the position of Rhizomys, the junction of the two long flexors in the sole has been regarded by Dobson as characteristic of the Hystricomorpha, though I have found it in other animals. As this characteristic is present in Rhizomys, it is worth |
