189o.] The Cetacea. 611 
goids; no longtitudinal or transverse crests of 
the skull; Balenide. 
The family of the whalebone whales is represented by many 
species both recent and extinct. These fall into a number of 
natural genera, which display several affinities towards different 
extremes. Thus the fin-backs (Balanoptera) have developed 
speed through increased length of bgdy ; the humpbacks (Megap- 
tera, have developed especial length of the fore limbs, while the 
right whales (Balzena) have acquired a huge oral cavity and the 
. greatest length of whalebone. The fin-backs pursue and devour 
great numbers of fishes of small and medium dimensions, and 
their maw derives an especial capacity for containing them, 
through the presence of numerous expansible longitudinal folds 
of its inferior walls. The Balznz, on the other hand, take in 
enormous quantities of water, which contains their minute mol- 
luscous food, and so enjoy an especial advantage in this direction, 
BALENIDE are abundant in the Miocene, having an origin 
prior to that of the Dephinide. They would seem to have derived 
their descent from some form allied to the Squalodontidz, since 
their nasal bones are more elongated than those of the Odonto- 
ceti, and in Plesiocetus the superior cranial bones show some of 
the elongation of that family. The genera of Balenide differ as 
follows : 
I. Frontal and parietal bones elongated on the median line. 
Cervical vertebrz distinct ; Plesiocetus Van Ben. 
Frontal and parietal bones much abbreviated in the 
median line. 
A, Cervical vertebrze all distinct; fingers four. 
a, Numerous gular folds ; vertebral canal not enclosed ; A 
No coracoid; manus long; Megaptera Gray.” 
A coracoid; manus not elongate ; Cetotherium Brandt.” 
Mandible with a long angle ; coronoid large; Herpetocetus Van B. 
11 The external characters of Cetotherium and Herpetocetus are unknown. 
12 Poescopia Gray, Burtinopsis Van Ben. 
13 Eschrichtius Gray. Cetotheriophanes Brandt. 
