10 PROCEEDINGS OF THE NATIONAL MUSEUM vol.66 



the rostrum; proximally the vomer and the premaxillae contribute 

 to its formation. Slaills of those cetaceans which possess an elongated 

 beak have experienced various modifications in the relations of 

 their component parts, but nevertheless in all forms now known the 

 vomer has been lengthened and so placed as to afford a maxnnum 

 support for the rostral and cranial portions of the skull. The vomer 

 increases in width posteriorly and takes part in the formation ot 

 the lateral walls of the mesorostral gutter. The contact between 

 the vomer and either premaxilla is clearly discernible distally m 

 skulls with damaged beaks although their surfaces are so smooth y 

 mortised into one another proximally that their relations can only 

 be determined by making cross sections of the rostrum. The posterior 

 limit of tliis contact between the vomer and the premaxilla is near 

 the anterior margin of the presphenoid. On the base of the slaill 

 the vomer extends backward upon the basisphenoid. 



The mesethmoid does not rise to the level of the premaxillae. It 

 sheathes the dorsal and lateral faces of the presphenoid and thus 

 forms a partition between the nasal passages superiorly, fills in the 

 frontal fontanelle, and provides support for the nasals, and inci- 

 dentally for the vertex of the skull. No trace of a pair of passages 

 openino- between the ectethmoids and the mesethmoid, and leading 

 into the brain case could be found in any of the skulls examined. In 

 skulls of Diochotichus, Geterhinops, and Squalodon the mesethmoid 

 incompletely fills the frontal fontanelle, and as a result a pair of 

 relatively larae foramina are formed, through which the olfactory 

 nerves reached the respiratory passages. Abel" states that these 

 foramina are present in a skull of Eurhinodelphis longirostris. 



A slit-like anterior border for the nasal aperture is formed by the 

 close approximation of the internal margins of the premaxillae. Be- 

 cause of this horizontal expansion of the premaxillae, most of the 

 anterior end of the presphenoid as well as the nasal passages are 

 hidden from view. The presphenoid is a porous bone which forms 

 a plug across the proximal end of the mesorostral gutter, but does 

 not rise to the level of the premaxillae above. 



The dorsal surface of the skull is constituted almost entirely by 

 the maxillae and premaxillae: the nasals and frontals form the 

 vertex of the skull. The maxillary notches are shallow and rather 

 broad. From a dorsal view, the maxillae are seen to increase m 

 width from the tip of the rostrum posteriorly. When they reach 

 the maxillary notches they push back over the supraorbital processes 

 of the frontals and expand laterally to form the so-called frontal 

 plates of the maxillae. They attain their greatest width opposite 

 the large concavities on the premaxillae. These plates of the maxil- 

 la Abei.o., Les dauphins longirostres du Bold6rlen (MIocSne Bup§rieur) des environs 

 d'Anvers. Mem. Mus. roy. d'hist. nat. de Belgique, Bruxelles, vol. 2, pp. 171, 172, 1902. 



