22 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 76 



Parietal extending forward laterally beyond posterior level attained by 

 nasals and nasal branches of maxillaries and intermaxillaries; 

 occipital shield extending forward over level of orbit and beyond 

 anterior level attained by articular portion of squamosal ; frontal 

 scarcely or not exposed on vertex; expanded lateral (articular) por- 

 tion of squamosal relatively large, its under surface deeply concave 

 [supraorbital process of frontal abruptly depressed at base to a level 

 noticeably below that of dorsal surface of interorbital region; 

 rostrum tending toward breadth rather than depth ; mandible con- 

 spicuously bowed outward] Bal^nopterid^. 



Scapula with acromion and coracoid processes rudimentary 

 [Rostrum and frontal with general relationships as in Balcenop- 



tcra] (Megaptera only) Megapterin.?:. 



Scapula with acromion and coracoid processes well developed. 



BaLuENOPTERIN^ 



Rostrum approaching maximum development, triangular in 

 outline when viewed from above, its sides straight or slightly 

 and evenly curved ; telescoping of braincase nearly at maxi- 

 mum, the portion of frontal exposed on vertex of skull 

 narrow but evident Balcenoptern. 



Rostrum at maximum known development, its sides parallel 

 with each other through most of basal half, then rather 

 strongly curved to tip ; telescoping of braincase at maxi- 

 mum, the portion of frontal exposed on vertex of skull so 

 narrowed that parietal is essentially in contact with maxil- 

 lary, and occipital touches base of nasal .Sibbaldns. 



DETAILS IN THE TOOTHED CETACEA 



In general structure of tlie skull the toothed cetacea have departed 

 more widely than the baleen vv^hales from the ordinary mammalian 

 type. Except in members of the extinct family Agorophiidce the 

 anterior nares have been forced back over the choanse so that the 

 arrangement of the bones forming the nasal passage is no longer 

 essentially normal ; the proximal ethmoid region is completely ex- 

 posed from above (pi. i, fig. la), without over-roofing by the nasals 

 or by the median portion of the f rontals ; the palatine takes part in 

 forming the anterior wall of the narial passage. The maxillary 

 (pi. I, fig. lb) has no trace of a horizontal orbital plate, even, ap- 

 parently, in the peculiar genera just alluded to. The rostrum is 

 often developed into a slender beak widely different in character 

 from the corresponding part of the skull in any other mammals. 

 While the process of telescoping in this group has without exception 

 followed the general course whose main features have already been 

 described (p. 4, etc.), the two chie"f variations in this course appear to 

 be almost as clearly indicated as in the case of the baleen whales. 

 In one (pi. 5, figs. 4, 5 ; pi. 7, figs. 2, 4) the backward thrust of the 



