MAMMALIA. 425 
a groove of the socket rather than in a proper socket for each tooth. 
From this disposition they have but little adhesion, which a slight effort 
may displace. The number of teeth varies among individuals of the same 
species as well as among the species themselves, as if these organs hold but 
a secondary importance in the existence of the dolphins. ‘The spiracles or 
blow-holes, after traversing the upper jaw, unite without in a single orifice, 
which is in the form of a crescent, and is situated at the top of the head. 
Their organs of sense seem to be in equal number with those of the other 
mammals, although most of them are less developed. The eye is very 
small, and furnished with narrow eyelids deprived of eyelashes; the pupil is 
cordiform. ‘The external orifice of the ear is scarcely perceptible. The 
tongue is thick, short, smooth, susceptible of bus very little motion, and 
sometimes fringed on its margins. The sense of touch is rendered very 
obtuse by the fact of their having a smooth skin deprived of hairs, and the 
presence of a layer of fat underneath. The seat of the sense of smell is 
not yet known. Besides the tail and its fin, the dolphins have, as organs of 
motion, two pectoral fins, and often on the middle of the back a fold of the 
skin which has the appearance of a fin. 
The genus Delphinorhyncus is characterized by a vaulted head and an 
elongated and narrow snout, with or without conical and crooked teeth: 
An osteological character is found in the bony structure of the head. The 
D. micropterus is distinguished by its small dorsal fin and the want of 
teeth ; it was found only once at the mouth of the Seine. Its geographical 
range is unknown. The second species of the genus, D. coronatus, is the 
largest of the family. Its lower jaw is a little longer than the upper one; 
both are beset with small, conical, very acute teeth, more numerous on the 
lower than on the upper jaw. The dorsal fin is semi-crescent-shaped; the 
caudal forms a complete crescent. The head is rather small in proportion. 
There are two concentric yellow circles on the forehead, whence the 
specific name is derived. This species is very common in the Arctic seas 
around the island of Spitzbergen. A third species, very imperfectly known 
as yet, is D. frontatus, quite distinct from the two preceding. The jaws 
are very much elongated, narrow, and thin, provided with twenty-four to 
twenty-five teeth on each side of both jaws. The forehead is much 
vaulted ; the dorsal fin reduced to a mere fold of the skin. The pectorals 
are scythe-shaped, and the caudals concave. The blow-hole is immedi- 
ately above the eye; its convexity being turned towards the snout. The 
haunts of this species are not accurately known. 
The genus Delphinus is distinguished by the form of the snout being less 
elongated’ and broader than in the preceding. The species are numerous, 
and found in every ocean; none of them are of a considerable size. A dor- 
sal fin generally exists, a single species only wanting this appendage. The 
most common one, D. delphis (pl. 106, jig. 4), congregates in numbers of 
individuals, and is met with in the Mediterranean as well as in the Atlantic 
and northern seas and near the equator. Its history, however, is little 
known, although spoken of by the ancients. It never reaches a length of 
more than six or eight feet. Its large jaws are covered with thin lips sus- 
‘629 
