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Show 174 MR. F. G. PARSONS ON T H E [Jan. 14, surface of the pouch, and runs backward and upward under the ear to be lost on the back of the neck. W h e n this is dissected away, there is seen to be a deeper layer of muscle having exactly the same direction, and also coming from the surface of the pouch. By far the larger portion of the outer wall of the pouch is formed by the sterno-facialis muscle, which is especially thick at the orifice forming a partial sphincter; the fibres of this muscle run downward and backward to be inserted into the anterior half of the sternum, superficial to the pectoralis major. There is no panniculus in the inner wall of the pouch, which consists solely of skin covering the masseter and buccinator muscles, with the Fig- 8. Pouch-muscles of Heteromys. exception of one narrow slip which rises from the mental symphysis, runs round the inner side of the opening of the pouch, and when it reaches the lower margin turns sharply outward, superficial to the fibres of the sterno-facialis, to run to the side of the neck, where it joins the platysma. This decussation of the fibres guarding the outer and the inner side of the aperture gives a powerful sphincteric action by which the pouch can be closed at will. It is worth while to compare the anatomy of the pouch in the Geomyidae with that in Cricetus, and to notice that, whereas in the former the pouch is formed by an invagination of skin probably perforating the platysma, so as to leave some of |