SKELETON OF CETACEA. 



423 



ramus. In the adult male the disproportionate growth of this 

 part of the jaw leads to an excess of length of the symphysial part 

 beyond the rest of the ramus. It is coextensive with the dental 

 series, which consists, in each ramus, of twenty-seven teeth, 

 conical or ovoid, according to their state of developement and 

 usage : the smallest teeth are at the two extremes of the series. 

 In the young Cachalot they are conical and pointed, but become 

 obtuse by use, whilst progressive growth expands and elongates 

 the base into a fang, which then contracts, and is finally solidified 

 and terminated obtusely. The teeth are separated by intervals 

 as broad as themselves. In respect to their mode of implantation 

 they offer a condition intermediate between that of the teeth of 

 the Ichthyosaurus and Grampus, being lodged in a wide and 

 moderately deep groove, imperfectly divided into sockets, the 



285 



Skull, Balccna mysticetus. 



septa of which reach only about half-way from the bottom of the 

 groove. 



In a foetal Southern Whale (^Balccna australls), each frontal is 

 a transversely elongated slender triangle, with its base at the 

 frontal suture, which is a thick vertical symphysis, and its apex 

 at the superorbital ridge : the inferior angle of the base rests upon 

 the prefrontals and upon the sides of the expanded base of the 

 vomer. The frontals take a very small share in the formation of 

 the cerebral cavity. Their cranial surface forms a small concavity 

 at the back part of the base : a half-canal is continued forward 

 from the lower angle of this surface into tlie nasal cavity. Almost 

 the whole of the upper and outer surface of the frontals is over- 

 lapped by the parietals and occipitals, leaving a very narrow ex- 

 posed transverse strip across the upper part of the skull. Tlie 

 anterior border of each frontal is joined mesially with the nasal. 



