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Show 164 MR. F. G. PARSONS ON THE [Jail. 14, animals approach more or less closely to the Hystricomorphine arrangement. In Myoxus the type is essentially Sciuromorphine, there is a distinct central tendon and a tendinous arcade connecting the anterior bellies of opposite sides ; the two anterior bellies, moreover, are in contact in the middle line. The same arrangement is found in Gerbillus (where, however, the posterior bellies rise from the bulla tympani), in Cricetomys, in the Murinae (Mus rattus, M. decumanus, and M. barbarus), and in Rhizomys among the Spalacidae. In Cricetus, Microtus, Myodes, and Hydromys (Windle) the central tendon is reduced to a mere tendinous intersection as in the Hystricomorpha, but the two anterior bellies are still in close contact. In Bathyergus and Georychus among the Spalacidae a similar arrangement is found, but, as in all other cases, the double nerve-supply of the muscle is preserved. Heteromys approaches most nearly to the Hystricomorphine type, since the two anterior bellies are not in contact and there is no tendinous arcade; there is, however, a distinct constriction and tendon between the anterior and posterior bellies. Transverse Mandibular Muscle.-This muscle is present in all cases ; it is perhaps better developed in the Muridae than in the other families. When the mylo-hyoid comes far enough forward the transverse mandibular is superficial to it. Mylo-hyoid. -The mylo-hyoid is attached posteriorly to the tendinous arcade connecting the digastrics and to the hyoid bone. Anteriorly the two muscles usually form a V-shaped border, with the aperture of the V forward, and do not reach the symphysis. In Myoxus, Microtus, the Murinae (M. rattus, decumanus, and barbarus), and in Heteromys the muscle extends farther forward than in the rest and is then deep to the transverse mandibular. Genio-hyoid.-The two muscles of opposite sides tend to coalesce posteriorly as in the Sciuromorpha. Genio-hgo-glossus.-This has the usual attachments ; it is specially large in Rhizomys. Styloid Muscles.-The stylo-hyoid has the usual Bodeut arrangement in passing deep to the digastric. In Rhizomys these two muscles are closely blended, while in Georychus the blending seems to be more complete, for no distinct stylo-hyoid could be made out. The stylo-glossus rises by tendon from the posterior part of the bulla ; it is very well marked in Cricetomys, while in Georychus and Bathyergus it rises by tendon from the stylo-hyal element of the hyoid arch. As in other rodents the stylo-pharyngeus was not seen as a distinct muscle. Sterno- and Cleiclo-mastoid.-In the Myomorpha these two muscles are perhaps not quite so distinctly separated one from another as in either of the other suborders. In Cricetus, Myoxus, Microtus, Myodes, Mus barbarus, M. rattus, and Heteromys the cleido-mastoid rises from the inner part of the bony clavicle under cover of the clavicular insertion of the trapezius ; it is inserted into the curved line of the occipital bone, close to the |