THE QUARTERLY REVIEW OF BIOLOGY 



of the foramina for the olfactory nerves 

 and overspread the internal borders of the 

 more or less vertical surfaces of the 

 frontals. 



In the skull of Zarhacbis flagellator, 

 the mesethmoid likewise provides some 

 support for the vertex of the skull, but 

 each ectethmoid has grown around the 

 passage for the olfactory nerve and has 

 overspread the lateral surface of the 

 mesethmoid, forming a crescentk foramen. 

 The curved rostral edge of each ectethmoid 

 is the original external edge, and at the 

 caudal end of the foramen is the original 

 internal edge. Succeeding stages in the 

 telescoping of the odontocete skull 

 brought about a further compression of 

 the fore wall of the braincase, causing the 

 mesethmoid to lose its original function 

 of supporting the vertex, and ultimately 

 it came to overspread the frontals below 

 the nasal bones on the posterior wall of 

 the narial passages. In skulls of living 

 porpoises the plate-like bone that sheathes 

 the anterior surfaces of the internal 

 borders of the frontals, conceals the 

 frontal fontanelle, and extends upward 

 to overspread the lower borders of the 

 nasal bones, unquestionably represents 

 the combined mesethmoid and ecteth- 

 moids, and corresponds to the cribriform 

 plate of other mammals. In most 

 mammals, the ethmoid plate ossifies into 

 a median mesethmoid bone bounded 

 below on either side by the ectethmoids. 

 These ethmoid bones comprise the cribri- 

 form plate, and in the early stages when 

 the mesethmoid and ectethmoids are 

 distinct the olfactory nerves pass out 

 between them. 



In a skull of a young Tursiops truncatus, 

 the porous plug-like presphenoid rests 

 in the trough of the vomer and the thin 

 lateral walls of the latter embrace the 

 lower half of this bone. Dorsally there 

 is a second thin curved bone, coextensive 



with the internal and posterior walls of 

 the narial passage, which fits into a groove 

 on the upper margin of the thin lateral 

 wall of the vomer and in addition sheathes 

 the inner borders of the frontals outside 

 the frontal fontanelle. One of these bones 

 is present in each narial passage. These 

 bones fulfill the requirements of the 

 ectethmoids. The inner margins of the 

 lateral plate-like ectethmoids do not 

 meet the mesethmoid above the pre- 

 sphenoid as in an older individual. The 

 mesethmoid no longer supports the vertex 

 of the cranium and has been crowded out 

 of its original position to overspread the 

 anterior wall of the braincase below the 

 nasal bones. Inferiorly on each side it 

 has fused with the corresponding ecteth- 

 moid. On the internal margin of each 

 ectethmoid is an incomplete fissure or 

 vestigial foramen, which marks the 

 original position of the orifice for the 

 olfactory nerve. In older individuals 

 these ethmoid bones completely fuse and 

 form a continuous sheet of bone, which 

 extends upward toward the vertex, over- 

 spreading the lower borders of the nasal 

 bones. The olfactory nerves were thus 

 effectively shut off from the narial passages 

 by the development of this bony plate. 



ACOUSTIC SENSE 



Life in the water necessitated a remodel- 

 ing of the organ of hearing so that it 

 would function under the changed condi- 

 tions. The initial stages in the trans- 

 formation of an organ of hearing designed 

 for the perception of air-borne sounds to 

 one adapted to receive water-borne sounds 

 are unknown, for the earliest known 

 zeuglodonts have the osseous portions of 

 this organ as highly developed as any of 

 the living whales. The ancestors of the 

 living cetaceans must have had functional 

 ear drums just as have man and other 

 mammals, but in the course of geological 





