BALENIDE 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.+ 
Tympanics, vertebre, 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. definita (sibbaldina) being apparently 
nearly related to the existing B. sibbaldi. A caudal vertebra from 
the Upper Eocene of Hampshire has been referred to Balenoptera, but 
does not afford sufficient evidence to prove the existence of the 
genus at that date. 
Extinct Genera.—The extinct genus Cetotherium of the European 
Pliocene may be taken to include a number of fossil Whalebone 
Whales allied to the Balenopterine 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 
Balenoptera. 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 
vertebre is also greater. In the typical forms (eg. C. brialmonti 
and C. dubiwm) the mandibular condyle is simple; but in C. 
(Heterocetus) brevifrons it is furnished with a projecting posterior 
talon, as in the Sperm Whale. 
Herpetocetus 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 
Meguptera, but has the greater part of the cavity filled by bone. 
There is a talon to the condyle of the mandible. 
Paleocetus, as-already mentioned (p. 232), is founded upon the 
ankylosed cervical vertebre 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 Balenidw it indi- 
cates a Right Whale. 
1 See P. J. Van Beneden, ‘‘ Histoire Naturelles des Balénoptéres,” A/ém. Acad. 
Belgique, xli. 1887, 
