428 ZOOLOGY. 
fig. 1) the genus would rest upon the presence of two dorsal prominences, 
one situated near the fin and the other near the head. The form of the 
snout is unknown, so that we are in doubt as to whether it is elongated as in 
the dolphin, or truncated as in porpoises. The naturalists of the “ Uranie” 
met a second species in 1819, between the Sandwich Islands and New South 
Wales, in 5° 28’ north latitude. This is known also under the name of 
O. rhinoceros. bac, 
Fam. 4. HETERODONTIDA, is difficult to characterize on account of the 
dissimilarity between the few genera and species of which it is composed, 
and the little knowledge possessed of them. The teeth are of different sorts, 
always few in number, and sometimes even wanting in the adult state. 
~The extinct genus Ziphius, which we place here for the sake of conveni- 
ence, ls intermediate between Hyperoodon and Physeter, and seems to have 
preceded and foreshadowed in the tertiary epoch these two genera of our 
days. Three Kuropean species are described, but none of them are fully 
known. ; 
The genus Myperoodon is based upon a singular structure of the head, 
which consists of a large and elevated apophysis rising from the middle of 
each upper maxillary, giving to the posterior part of the head that peculiar 
shape which characterizes it. Their teeth, two in number, small, acute, 
near the extremity of the jaw, sometimes hidden under the gum or wanting, 
whence the name of Delphinus edentulus, and the generic appellation of 
Aodon. The absence of teeth has led some others to place this animal 
among whales proper, but as it has no whalebones others have been 
induced to bring it nearer the dolphins. Only one species is described as 
belonging to this genus, the H. baussardiw from the northern Huropean 
seas. It has a dorsal fin and small pectorals. The beak is detached from 
the forehead, which is rounded and abrupt. The eyes are quite distant 
from the angles of the mouth, a peculiarity which distinguishes it from 
the dolphins proper. The body is fusiform, and tapers rapidly away from 
the dorsal fin to the tail. 
The genus Heterodon must provisionally include the two-toothed cachalot 
of Sowerby (Physeter bidens sowerbyt, Dewh.), which we shall designate by 
the name of H. sowerbyi. Its characters consist of a narrow and elongated 
snout, the upper jaw longer than the lower, which receives it; one single 
tooth below on each side, compressed, and obliquely directed backwards, 
placed in the middle of the margin of the jaw. A blow-hole, crescent- 
shaped, with the concavity forwards. This species was observed on the 
coast of England. The two-toothed dolphin (Delphinus bidens) resembles 
the Hyperoodon by the form of its head. Its place is, at any rate, in this 
family; but whether it belongs to the genus Heterodon, or to any other, 
we cannot decide at present. 
We place here the genus Arnanacus of Lacépéde, created for one species 
of cetacean, called by the Greenlanders Arnanak, from the supposed purga- 
tive qualities of its flesh and fat. It has also been referred to the genus 
Monodon, and is the M. spurius of the ‘‘ Encyclopédie Méthodique.” The 
Arnanicus grenlandicus has one or two small obtuse, conic, and crooked 
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