20 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 72 



In the genera of the section Balccnopterini the intermaxillary and 

 also the maxillary are rather strongly flattened anteriorly so that the 

 facial part of the skull has lost its primitive pointed form. Body and 

 tail are uncommonly long, adapted to more rapid swimming with 

 stronger muscles. Among the alterations produced by these muscles 

 are the higher spinous processes on the dorsal and caudal vertebrae. 

 The hand is shaped more like an oar blade. The fingers are laid more 

 closely together and the third and fourth may have the number of 

 phalanges increased ; the first digit has completely disappeared. The 

 skeleton of the hind limb is more reduced. The mouth is modified 

 in its own way ; its outbowing in the upward direction is slight or 

 absent, that to the sides and downward is conspicuous. The floor of 

 the mouth cavity has become to a high degree expansible, and the 

 intermaxillary and maxillary, like a broad, more or less flattened lid, 

 cover . over the pouch which it forms. The Balsenopterines stand 

 lower than the known Baleenines in the condition of the cervical verte- 

 brae : the bones retain their freedom. 



Among the known Balsenopterines, Rhachioncctes is one of the most 

 primitive. Its nasal bone is still relatively very well developed. The 

 breadth of the intermaxillary and maxillary in front is rather slight. 

 The supraorbital process is relatively weak and not strongly flattened. 

 The braincase is relatively only slightly telescoped so that on the 

 middle of its upper side there can be seen not a little of the frontal. 

 The supraoccipital is not especially large or forward-slanting. The 

 articular surface for the lower jaw on the squamosal is not pushed 

 out especially far downward and backward, and, when seen from 

 beneath, has not entirely covered the mastoid or pushed the exoccipital 

 very far backward. Bony palate relatively not strongly lengthened 

 behind. The point of attachment for the neck muscles on the basal 

 part of the occipital is still tubercular, and the basioccipital on the 

 whole is only to a slight degree shaped to accommodate the larynx 

 and gullet. In contrast with its nearest recent allies Rhachionectcs 

 stands lower in a few other respects also : an evident capitulum is 

 still found on some of its anterior ribs ; the skin beneath its mouth 

 cavity is not thrown into longitudinal folds; the dorsal fin is not 

 present ; the hand is relatively short, and the number of phalanges is 

 only a little increased. It has perhaps high specializations in its 

 decidedly heavy under jaw, which slightly suggests Neohalccna, and 

 in its somewhat upwardly arched facial portion of the skull. 



Plesiocetus from the Tertiary of Europe, and presumably from that 

 of North America also, is best known from the skull. To a high 



