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Show 1894.] MYOLOGY OF RODENTS. 257 In Pteromys the sterno-hyoid is inserted by a thin tendon. Omo-hyoid.-The omo-hyoid may be present or not. When it is present it has the human attachments to the hyoid bone and anterior border of tbe scapula, but it never has any tendinous interval; it runs, moreover, a straight course between its two attachments. In the Dipodidae it is present (D. cegyptius, D. hirtipes, Alectaga indica; in the last it is specially well marked). In the Octodontidse it is not constant. In Octodon and Capromys it is feebly developed, in Myopotamus it is absent, while in Aulacodus it is well marked. In tbe Hystricidae it is well marked in Sphingurus and Erethizon1, but in Hystrix it was seen as a thin layer of muscular fibres disappearing in the fascia under the sterno-mastoid. In the Chinchillidae and Dasyproctidae it is absent (Chinchilla, Lagostomus, Dasyprocta, Ccelogengs). In the Caviidae it is also absent (Cavia cobaya, Ceredon rupestris, Dolichotis). In the Sciuromorpha it is always present (Sciurus, Pteromys, Xerus, Spermophilus, Arctomys, Castor). Levator Claviculce.-The levator claviculae rises either from the basioccipital behind the origin of the scalenus anticus, or from the ventral arch of the atlas; it is inserted chiefly iuto the acromion and metacromion and the fascia of the shoulder, but sometimes it extends to the acromial end of the clavicle. It is supplied by branches from the cervical plexus. In the Dipodidae it rises from the front of the atlas and runs to the outer end of the clavicle. In the Octodontidae it is not constant. In Aulacodus, Capromys melanurus 2, and Myopotamus it rises from tbe basioccipital, but in Octodon from tbe atlas. In the Hystricidae it rises from the atlas in Sphingurus and Erethizon 3, but in Hystrix it comes from the base of the skull aud is inserted into the acromion and fascia of the arm as low as the elbow. (See fig. 10.) In tbe Chinchillidae it rises from the atlas in Chinchilla, and from the basioccipital in the Viscacha; in the latter animal it is inserted into the outer half of the clavicle as well as the metacromion. In the Dasyproctidae it rose from the atlas in the specimen of D. cristata which I dissected ; in the specimen dissected by Mivart and Murie it rose from the basioccipital4. In Ccelogenys it rises from the basioccipital and is very large. In the Caviidae it rises from the basioccipital in C. cobaya, Ceredon rupestris, and Dolichotis. In all the Sciuromorpha it rises from the atlas and is inserted into the metacromion, never seeming to reach the clavicle. In 1 Journal of Anatomy, vol. xxii. p. 126. 2 Dobson, P. Z. S. 1884, p. 234. 3 Mivart, P. Z. S. 1882, p. 271. 4 P. Z. S. 1866, p. 383. |