NO. 5 TELESCOPING OF THE CETACEAN SKULL 21 



• Entire skull when viewed from the side arched more abruptly at 

 base than in rostral portion, the general curve therefore not 

 approximately following the arc of a circle ; length of skull con- 

 tained about three times in length of vertebral column ; depth of 

 skull from highest part of arch to lower border of mandible less 



than greatest depth of thorax Eubalcena. 



Entire skull when viewed from the side arched rather uniformly in 

 a curve which approximately follows the arc of a circle ; length 

 of skull contained about one and one-half times in length of 

 vertebral column; depth of skull from highest part of arch to 

 lower border of mandible conspicuously greater than greatest 



depth of thorax Bahvna. 



Supraorbital wing of frontal broad, its antero-posterior diameter at 

 middle more than one-half its transverse diameter; rostrum so 

 slightly arched that a straight line can be drawn from its extremity 

 to its base without passing outside of the general contour ; general 

 outline of rostrum when viewed from above tapering rapidly from a 

 broad base to slender tip ; most of the ribs free from the vertebrae ; 



lumbar vertebrae 2 {Neohalcena only) Neobal.i:nid.^. 



Telescoping of skull accomplished by a combined forward movement of 

 posterior elements and backward movement of anterior elements which 

 produces at least some obvious indication of interdigitation between 

 rostral and cranial elements ; nasals and nasal branches of premaxillaries 

 not situated entirely anterior to the level of the orbital wings of the 

 frontals ; a definite " nasal process " of the maxillary always present 

 (Balaenopteroids). 

 Parietal entirely behind posterior level attained by nasals and nasal 

 branches of maxillaries and intermaxillaries ; occipital shield not 

 extending forward over level of orbit or beyond anterior level 

 attained by articular portion of squamosal ; frontal broadly exposed 

 on vertex; expanded lateral (articular) portion of squamosal rela- 

 tively small, its under surface not deeply concave. 



Supraorbital process of frontal sloping gradually downward and 

 outward from level of dorsal surface of interorbital region ; 

 parietals coming in contact or nearly so on vertex between 

 occipital shield and frontal; nasals small (normal), their com- 

 bined dorsal area equal to much less than half that of supra- 

 orbital portion of frontal ; rostrum tending toward breadth rather 

 than depth ; mandible slender, conspicuously bowed outward 



(Cetotherhim and related genera) Cetotheriid.e. 



Supraorbital process of frontal abruptly depressed at base to a 

 level noticeably below that of dorsal surface of interorbital 

 region ; parietals not coming in contact or nearly so on vertex 

 between occipital shield and frontal ; nasals greatly enlarged, 

 their combined dorsal area equal to more than half that of 

 supraorbital portion of frontal ; rostrum tending toward depth 

 rather than breadth ; mandible heavy, slightly bowed outward 

 (Rhachianecfes only) Rhachi.\nectid.e. 



