FIG. 19. Dipodomys merriami AMNH-M 182081, caudal cranium in left lateral (rev.) (A) and dorsal (B) aspects; certain sutures emphasized to improve visibility. Names for middle-ear spaces and sutural bounds after Webster (1975). Insignificant rod (small black arrows in A, suprmeat sp squ in B) running across surface of multipartite middle ear (hypotympanum, epitympanum, antrum) is all there is of squamous portion of squamosal in this heteromyid. Rod ends freely over external acoustic meatus, and is nowhere fused to cranial sidewall. In A, black asterisk lies on equally insignificant "body" of squamosal. Apparent suture (large white arrows) crossing caudal bulge of immense middle ear in A and B marks track of an internal "mastoid" septum, seen through translucent walls. Roof of external acoustic meatus is formed by ectotympanic, whose