. Bulletin - United States National Museum. Science. THE SPERM WHALE OEYCTEROCETUS 51 The opposite premaxillaries of all known fossil and recent physeteroids are dissimilar in form. The upper surface of the right premaxillary in front of the premaxillary foramen is concave for a distance of about 150 mm. on two skulls (USNM 14729 and 14730) and convex on the other skull (USNM 22926). As a result of telescoping, the right pre- maxillary is extended backward on the upper surface of the cranium to the posterior borders of the maxillaries. Posterior to the nasal passages the right premaxillary ex-
. Bulletin - United States National Museum. Science. THE SPERM WHALE OEYCTEROCETUS 51 The opposite premaxillaries of all known fossil and recent physeteroids are dissimilar in form. The upper surface of the right premaxillary in front of the premaxillary foramen is concave for a distance of about 150 mm. on two skulls (USNM 14729 and 14730) and convex on the other skull (USNM 22926). As a result of telescoping, the right pre- maxillary is extended backward on the upper surface of the cranium to the posterior borders of the maxillaries. Posterior to the nasal passages the right premaxillary ex- pands into a broad, thin plate which is applied to the upper surface of the frontal, overlapping the maxillary externally and posteriorly. A large foramen pierces the right pre- maxillary anterior to the level of the maxillary incisure. On its internal border behind the nasal passages the right premaxillary overrides and partially conceals the flattened nasal bone except for a maximum 53 mm. wide exposure of its external border. The left premaxillary has been pushed outward by the enlargement of the left nasal passage and terminates 115 to 130 mm. behind the level of the posterior wall of this passage. No foramen is present on the left premaxillary anterior to the nasal passages. The inner borders of the premaxillaries are closely approxi- mated in front of the presphenoid but do not completely roof over the mesorostral gutter on any one of the four fossil skulls. Anterior to the widest basal portion of the rostrum (fig. 30), the upper surfaces of the premaxillaries become progressively more convex and on USNM 22931 comprise one half or more of the vertical diameter on the distal 280 mm. of the lateral surface of the rostrum. Anterior to the widest basal portion of the rostrum the downward slope of the upper surface of the premaxillaries becomes more pronounced and is accentuated near the extremity. The premaxillaries (pi. 28 top) by themselves comprise the distal 75 mm. of th
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