2.2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 72 



II. Intermaxillary and maxillary broad anteriorly, flattened. 

 BalcBnopterim. 



A. Nasal relatively well developed. 



Rhachionectcs. 



B. Nasal reduced. 



1. Area of insertion of neck muscles on basal part of 



occipital tubercular. 

 a. Articular surface for lower jaw on squamosal 

 not strongly pushed backward, not covering 

 the mastoid when seen from below. 

 PlcsiocetHS. 

 h. Articular surface for lower jaw on squamosal 

 more pushed backward, covering the 

 mastoid. 



Cetotheriuin. 



2. Area of insertion of neck muscles on basal part of 



occipital compressed. 



a. Hand not especially elongated. Shoulder 



blade with crest. 

 Balcrnoptera. 



b. Hand greatly elongated. Shoulder blade 



without crest. 

 Megaptcra. 



Squalodontidae. — The Squalodonts must have originated from 

 among the most primitive Balsenids which still had the teeth shaped 

 like those of the Zeuglodonts but increased in number, and which had 

 not yet begun to get the mouth refashioned into a catching-bag. Their 

 difi^erences from the most primitive Balsenids are due especially to 

 stronger pressure of the water on the facial part of the skull ; most 

 likely the Squalodonts were from the beginning more rapid swimmers 

 than the Balsenids. The nasal passage is pushed much further back, 

 not by muscle action alone, but probably especially by the influence of 

 the facial adipose cushion. The water both stimulates the cushion 

 to growth and presses it against the nasal passage. The nasal bone 

 is completely atrophied, almost tubercular in form, and pressed into 

 the frontal in the fore wall of the braincase, not or almost not cover- 

 ing over any part of the nasal cavity. The plates of the ethmoid are 

 probably pushed wholly away and the lamina cribrosa has probably 

 become a solid bone-plate without perforations or almost so. The 

 nose muscles, the pneumatic diverticulum from the nasal passage, the 

 adipose cushion of the snout, in short all that covers the skeleton of 



