MAMMALIA. 425 



a gi'oove of tlie 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 ninong the species themselves, as if these organs hold but 

 a secondary importance in the existence of the dol|3hins. The s]3iracles 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 puj)il is 

 cordiform. The external orifice of the ear is scarcely perceptible. The 

 tongue is thick, short, smooth, susceptible of but 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 

 rahge is unknown. The second species of the genus, D. coronatus, is the 

 largest of the famih\ Its lower jaw is a little longer than the upper oi»; 

 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 Delphmus 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 {pi. 106, fig. 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 



