BALAZNIDAZ 245 



out. Nearly all the individuals hitherto examined with any care, 

 whether from the North Pacific, the Australian seas, or the Indian 

 Ocean, come very near in structure to one or the other of the 

 Atlantic forms described above, so much so that some zoologists 

 have been induced to believe that there are but four species, each 

 of which has a wide, almost cosmopolitan range, while others have 

 described and named almost every individual specimen captured as 

 belonging to a different species. 1 



Tympanies, vertebra?, and other bones of Rorquals are among 

 the commonest cetacean remains found in the Pliocene Crags of 

 England and Belgium. Several species, varying in dimensions, are 

 known from these deposits, B. definite (sibbaldina) being apparently 

 nearly related to the existing B. sibbaldi. A caudal vertebra from 

 the Upper Eocene of Hampshire has been referred to Bakmoptcra, but 

 does not afford sufficient evidence to prove the existence of the 

 genus at that date. 



Extinct Genera. — The extinct genus Cctotherium of the European 

 Pliocene may be taken to include a number of fossil Whalebone 

 "Whales allied to the Balamopterine group, several of which have 

 been described under other names, such as Plesiocetus, Heterocetus, 

 and Amphicetus. They are readily characterised by the form of 

 the tympanic bone, which is much narrower in front than behind, 

 the roughened inferior surface being in the shape of an isosceles 

 triangle, and the notch for the Eustachian canal being smaller, and 

 descending nearer to the inferior border of the inner wall than in 

 Balcmoptera. The skull is longer than the latter, with a greater 

 interval between the occiput and the frontal, and with longer and 

 more flattened nasals. The relative thickness of the cervical 

 vertebra? is also greater. In the typical forms (e.g. C. brialmonti 

 and C. dubiuni) the mandibular condyle is simple ; but in C. 

 (Heterocetus) brevifrons it is furnished with a projecting posterior 

 talon, as in the Sperm Whale. 



IB ipetocetus is known by a comparatively small species from the 

 Belgian and English Crags, characterised by the extreme inflation 

 of the egg-shaped tympanic bone, which approximates to that of 

 Megaptera, but has the greater part of the cavity filled by bone. 

 There is a talon to the condyle of the mandible. 



Baheocetus, as already mentioned (p. 232), is founded upon the 

 ankylosed cervical vertebra? of a small Whale originally considered as 

 having been derived from the Kimeridge Clay, but which doubtless 

 came from the Suffolk Crag ; if it belongs to the Balcenidce it indi- 

 cates a Eight Whale. 



1 See P. J. Van Beneden, "Histoire Naturelles des Balenopteres, " Mem. Acad. 

 Belgique, xli. 1887. 



