HISTORY OF WHALES 



zoi 



bones are missing, but the fontanelle is 

 quite large and the anterior wall of the 

 braincase is unusually thick. The skull 

 of Archaeodel-phis patrius on the other hand 

 has distinct nasal cavities in front of 

 olfactory foramina of moderate size, 

 which are set off from the forward sloping 

 narial passages. The few endocranial 

 casts of extinct odontocetes that have been 

 described show that at the beginning of 

 the Miocene the olfactory organs had 

 dwindled to very small proportions as 

 compared to Prozeuglodon. In the case of 

 Cyrtodelphis s-ulcaMs (Dal PiaZ, 1905) and 

 Prosqualodon davidi (Dart, 192.3), the 

 olfactory lobes are attached in a nipple- 

 like fashion to the cerebral hemispheres 

 and the insertion is somewhat higher 

 than in "Zeuglodon." All of the squalo- 

 donts now known retained remnants of 

 olfactory structures, if our interpretations 

 of the foramina on the fore wall of the 

 braincase are correct. Other extinct 

 Miocene porpoises, including the primi- 

 tive ziphioid types Diochotichm and Squalo- 

 delfhis, have a pair of foramina on the 

 anterior wall of the braincase for the 

 passage of sensory nerves. 



The living toothed whales have lost 

 their olfactory organs, though such struc- 

 tures have been found in foetuses. Changes 

 in the relations of the bones in the region 

 where the olfactory nerves originally 

 found passage have resulted in the closure 

 of the opening in the cribriform plate and 

 the concomitant lateral expansion of its 

 component parts, the mesethmoid and 

 ectethmoids, to completely cover the 

 fontanelle on the anterior wall of the 

 braincase between the medial margins of 

 the frontal bones. Mechanical changes 

 in the relations of the component parts of 

 the odontocete type of skull appear to have 

 restricted at first and finally prevented 

 the physiological functioning of the 

 olfactory apparatus. These changes have 



hastened the final reduction of the dwin- 

 dling olfactory apparatus of the toothed 

 whales. On the basis of skulls of Miocene 

 porpoises, one would be led to conclude 

 that the passages for the olfactory nerves 

 were reduced by the upward and lateral 

 expansion of the ectethmoids. In many 

 of the late Miocene porpoises and in 

 practically all of the living types, the 

 mesethmoid and ectethmoids have fused 

 to form a continuous sheet of bone, which 

 overspreads the area between the nasal 

 bones and the lateral margins of the 

 trough-like vomer, forming the posterior 

 wall for each narial passage. The general 

 course of the reduction of the olfactory 

 organs can be demonstrated also by 

 comparison of brain casts of fossil por- 

 poises from successive geological stages. 



The position of the mesethmoid with 

 reference to the frontals in certain well 

 known types of extinct porpoises is 

 especially instructive and furnishes data 

 from which one may visualize the steps 

 that culminated in the conditions found 

 in skulls of living porpoises. The transi- 

 tion from one stage to another can not 

 be traced at present in a single phyletic 

 series, and recourse must be had to several 

 unrelated types of porpoises. The brief 

 review that follows is necessarily incom- 

 plete, but it seems likely that the changes 

 in the relations of the mesethmoid and 

 ectethmoids under discussion clearly indi- 

 cate the successive stages in the closure of 

 the foramina for the olfactory nerves. 



In Squalodoiz the mesethmoid forms the 

 most dorsal portion of the partition 

 between the narial passages, divides the 

 fontanelle between the frontals on the 

 fore wall of the braincase into two large 

 foramina through which the olfactory 

 nerves pass and, extending upward, under- 

 lies the nasal bones and provides addi- 

 tional support for the vertex of the skull. 

 The ectethmoids form the external surfaces 



