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Show 1896.] MYOLOGY OF RODENTS. 187 The union of the tendons in the sole has been already alluded to as not being entirely confined to the Hystricomorpha. In the following points the Dipodidae resemble the Myomorpha :- 1. The sciuromorphine arrangement of the digastric. 2. The presence of a transverse mandibular muscle. 3. The absence of the scapulo-clavicularis. 4. The presence of the omo-hyoid. 5. The absence of the splenius colli. 6. The origin of the levator claviculae (acromio-trachelian) from the atlas. The first three of these are very important and constant points, the latter three are sometimes noted in the Hystricomorpha. O n the whole I think that the myological points in favour of myomorphine tendencies for the Dipodidae are far stronger than those in favour of hystricomorphine. GENERAL SUMMARY OF MUSCLES OF RODENTS. In order to complete my paper I propose to give a series of lists of the different points in which the four suborders of Rodents differ from and resemble one another, though the following pages are only tentative, and liable to require rearrangement as further material is added. They may, however, prove useful in directing the attention of future observers to the muscles deserving of special notice from a classificatory point of view7, and they may also be of service in showing the muscles that are constant in Rodents nearly related, whatever their mode of life may be, and that may turn out to be equally constant in nearly related groups belonging to other orders. In a former contribution1 the differences between the Hystricomorpha and Sciuromorpha are summarized. Differences between the Myomorpha and Hystricomorpha. 1. The part of the masseter which passes through the infraorbital foramen is usually small in the Myomorpha, large in the Hystricomorpha. 2. The Myomorpha have the sciuromorphine arrangement of the digastric. 3. The transverse mandibular muscle is present in the Myomorpha, absent in the Hystricomorpha. 4. The omo-hyoid muscle is always present in the Myomorpha, and may be present or absent in the Hystricomorpha. 5. The acromio-trachelian (levator claviculae) always rises from the arch of the atlas in the Myomorpha. In the Hystricomorpha it sometimes rises from the basioccipital. 6. The scalenus anticus is usually absent in the Myomorpha, usually present in the Hystricomorpha. 1 P.Z.S. 1894, p. 294. |