290 ANNALS NEW YORK ACADEMY OF SCIENCES 



g. Prominent rugosites upon the supraoccipital, pterygoid and 

 basioccipital bones of the skull. 



li. Compressed tympanic bulla? having concave internal borders. 



4. Cervical vertebrae entirely free and showing no evidences of anky- 

 losis between members of the series. 



5. Atlas and axis possessing massive, rugose neural arches; axis with 

 comparatively small foranyna through the wing-like transverse processes. 



6. Ribs possessing tubercles, necks and heads as far back as the eighth, 

 and in these portions resembling an Odontocete. 



7. A long and straight humerus of the Plesiocetus type. 



8. Very large pelvic elements, the presence of a large foramen and the 

 comparatively slight reduction of the pubis and ischium. 



Relationship of Rhachianectes 



Rhachianectes glaucus is apparently not closely related to any of the 

 existing baleen whales, but in some respects it stands intermediate be- 

 tween the Balseninse and Balasnopterinae, but nearer the latter. In many 

 •skull characters, it approaches closely the Pliocene whales of the genus 

 Plesiocetus which is allied to the existing Balsenopterinae ; in fact, were it 

 not for its specialized mandible, it must certainly be considered as nearly 

 related to them. The fossil whales of the Plesiocetus group possessed 

 mandibles having the proximal portion of each ramus, internally, widely 

 concave and leading into a large dental canal; in short, much as in the 

 mandibles of the existing toothed whales. Rhachianectes, however, al- 

 though resembling Plesiocetus in many important skull characters, pos- 

 sesses a specialized mandible similar to that of the Right Whales ; that is, 

 the proximal portion, internally, is not concave, and the dental canal is 

 small. This type of mandible prevents the phylogenist from taking 

 Rhachianectes off from the Plesiocetus group, unless he wishes to con- 

 sider that while persisting until the present day with comparatively little 

 modification of its primitive skull characters, it has undergone consider- 

 able specialization of the mandible alone. This is a perfectly possible 

 supposition, which I am inclined to believe is true, since Rhachianectes 

 shows such marked affinities to Plesiocetus in skull characters and is so 

 strongly separated from the other known genera of fossil and recent 

 whales. It is, upon the whole, one of the most remarkable of existing 

 cetaceans and might be called a "living fossil." 



The Section then adjourned. 



William K. Gregory, 



Secretary. 



