GENERAL CHARACTERS. 3 
to be the case. It may be noted here that the few Cetaceans inhabiting fresh- 
water are somewhat less markedly different in structure from ordinary mammals 
than are the marine representatives of the order; and this leads the writer last 
cited to conclude that Cetaceans were first modified for the assumption of a purely 
aquatic life from land mammals frequenting the banks of rivers, and that after 
having acquired natatorial powers in fresh water, their ancestors subsequently 
migrated to the sea, where they have attained their present remarkable develop- 
ment. 
Before making any further remarks on Cetaceans in general, it 
will, however, be convenient to take into consideration the leading 
structural features by which they are distinguished from other mammals. As 
already mentioned, Cetaceans are characterised by their fish-like form, the head 
passing imperceptibly into the body without any distinct neck, and at the other 
extremity the trunk gradually tapering to the tail, which terminates in a forked, 
horizontal fin-ike expansion, commonly known as the “flukes.” ‘The head is 
large in proportion to the body, with a wide mouth, often furnished with a few 
bristles, at least in the young state. The fore-limbs are represented by flippers, 
encased in a continuous skin, showing no outward indications of digits, and without 
the shghtest trace of nails or claws, while of the hind-limbs there is not the least 
external trace. With the exception of the aforesaid sparse bristles in the neigh- 
bourhood of the mouth, the smooth and shining skin is entirely devoid of all traces 
of hair, neither has it any structure corresponding to the seales of fishes. Beneath, 
it is underlain by the thick layer of oily fat termed “blubber,” the function of 
which is to prevent the absorption of the heat of the body by the water. The 
majority of the species have a fin on the back, which is entirely composed of 
integument, and has no internal skeleton corresponding to the similarly situated 
fins of fishes. The eye is extremely small; and, as in fishes, the ear has no external 
conch, and opens by an exceedingly small aperture behind the eye. The nostrils, 
which may have either a single or double opening, are situated on the very summit 
of the head, and thus reach the surface of the water before any other part when 
the animals rise horizontally. 
In the skeleton the bones are remarkable for their loose and 
spongy structure; and in the living state are saturated with oil. 
The majority of the species have teeth, which may be confined to the lower jaw, 
and these represented only by a single pair. In all cases these teeth are of a 
simply conical or compressed form; and in many of the dolphins they are much 
more numerous than in any other mammals. In no instance is there any replace- 
ment of the teeth; but Dr. Kiikenthal has discovered that in some species there 
are rudimentary successional teeth which never come to maturity, whence it is 
considered that the functional teeth correspond to the milk-series of those mammals 
in which there is a replacement. With regard to the great number of teeth present 
in some Cetaceans, the same investigator considers that this is due to the division 
of a number of trilobed teeth like those of certain seals (compare the figure in 
Vol. II. p. 142) into three parts, whereby three distinct teeth have been produced 
out of what was originally a single tooth. In the baleen, or whalebone-whales, the 
place of teeth is taken by the horny structure technically termed “baleen,” but 
Characteristics. 
Skeleton. 
