20 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 76 



maxillary/ and the occipital shield comes in contact with the maxil- 

 lary, premaxillary and nasal. A definite set of other characters is 

 associated with these final stages of telescoping. The straining-bag 

 formed by the mouth is increased in capacity not by an arching of 

 the rostrum as in the baljenoids, but by a bowing outward of the 

 lower jaws, a broadening and flattening of the rostrum, and a longi- 

 tudinal folding of the skin of the entire throat and underside of the 

 mouth to allow for great distention. While the jaw is not con- 

 spicuously increased in size relatively to the size of the skull the parts 

 of the cranium serving as its suspensorium are enlarged and special- 

 ized to a greater degree than in RacJiiancctes. The characters just 

 enumerated distinguish this group of genera as a family separate 

 from the four other families of baleen whales. Within the limits of 

 the group the humpbacks, Mcgaptera, are sharply contrasted with 

 the finbacks, Balcenoptera and Sibhaldus, by the unusual structure 

 of the scapula and by the great elongation of the manus. In the fin- 

 backs the scapula retains the form characteristic of the baleen whales 

 in general : the coracoid and acromion are large, functional processes. 

 In the humpbacks the processes are reduced to mere tubercles. Hump- 

 backs and finbacks differ from each other sufficiently to be regarded 

 as the representatives of two subfamilies. 



The more important characters of the genera and higher groups of 

 baleen whales are tabulated in the following key : 



KEY TO THE FAMILIES AND GENERA OF BALEEN WHALES 



Telescoping of skull accomplished chiefly by forward movement of poste- 

 rior elements ; intetdigitation of rostral and cranial elements of skull 

 absent or slight, the nasals and nasal branches of the intermaxillaries 

 situated entirely anterior to the level of the orbital wings of the frontals ; 

 no definite "nasal process" of maxillary ever present (Balsenoids). 

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

 middle less than one-third its transverse diameter ; rostrum so highly 

 arched that a straight line cannot be drawn from its extremity to any 

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

 general outline of rostrum when viewed from above attenuate ; most 

 of tiie ribs attached to the vertebrc'e; lumbar vertebrse 10 or more. 



Bal.enid;!?. 



^ In a 20-foot skull of Sibbaldus musculus (No. 49757, U. S. Nat. Mus.) the 

 upper edge of the parietal rises to the same level as the dorsal surface of 

 the maxillary; it is separated from the maxillary by a groove about 10 mm. 

 wide and 20 mm. deep, at the bottom of which can be seen the superior margin 

 of the frontal. 



