NO. 5 TELESCOPING OF THE CETACEAN SKULL 33 



■ KEY TO THE FAMILIES AND SUBFAMILIES OF 

 TOOTHED CETACEA 



Combined enlarging of brain and pushing together of braincase and anterior 

 part of skull so little advanced that a distinct trace of the primitive 

 postorbital constriction remains visible from above ; parietals in contact 

 on vertex behind frontals; maxillary sometimes (in Archaodelphis) 

 forming part of anterior rim of orbit [palatine normal in position, not 

 forming part of anterior wall of backward-sloping narial passage] 



(Agorophhis, Archaodclphis and Xenorophiis) Agorophiid^, 



Combined enlarging of brain and pushing together of braincase and 

 anterior part of skull so much advanced that no evident trace of the 

 primitive postorbital constriction remains visible; parietals not in contact 

 on vertex behind frontals ; maxillary never forming part of anterior rim 

 of orbit. 



Palatine normal in position, not forming part of anterior wall of 



slightly backward-sloping narial passage [Relation of maxillary to 



frontal such that the line of contact between these bones extends 



upward at a conspicuous angle from the anterior border of the orbit, 



leaving a broad area of the frontal exposed over middle of orbit when 



skull is viewed from the side ; parietal not visible as a separate 



element on side of braincase in adult ; palatine partly covered by 



pterygoid, its only surface exposure situated in the normal position 



on the roof of the mouth ; hindermost ribs supported by transverse 



processes which appear to be not serially homologous with those 



which support the others; teeth tusk-like in structure]. 



Facial depression greatly developed, obliterating the longitudinal 



ridge behind narial orifice ; brain relatively small, situated far 



behind orbit ; zygoma complete ; (Physeter only ; exact position 



of related extinct genera not certain) Physeterid^. 



Facial depression moderately developed, distorting but not oblit- 

 erating the longitudinal ridge behind narial orifice; brain rela- 

 tively large, extending forward to level of orbit ; zygoma incom- 

 plete ; (Kogia only; exact position of related extinct genera not 



certain ) KoGiiD^. 



Palatine not normal in position, forming part of anterior wall of 

 essentially vertical narial passage. 



Anterior teeth single-rooted ; posterior teeth double-rooted, their 



crowns flattened laterally and with serrate margins. .Squalodontid^. 

 Anterior and posterior teeth alike single-rooted, their crowns 

 rarely flattened laterally, and never with serrated margins. 

 Pterygoids completely covering the palatines on ventral aspect 

 of skull ; palatines widely separated from each other and 

 from median line of palate; external reduplication of ptery- 

 goid as large as original plate and closely simulating the 



original plate in form (Platanista only) Platanistid^. 



Pterygoids not completely covering the palatines on ventral 

 aspect of skull ; palatines in contact or virtually so in median 

 line of palate ; external reduplication of pterygoid, when 

 present, never as large as the original plate and never closely 

 simulating the original plate in form ; [Relation of maxil- 



