186 JURA. NATURAL HISTORY. 



differs from that species, not only in size but in other 

 important circumstances ; while its residence alone proves 

 it to be different from the fluviatilis, even if it did not 

 possess characters sufficiently distinct to form a new 

 and separate species. 



It is impossible, from an individual specimen, to de- 

 termine the average size of the species, as there is no 

 criterion by which to distinguish an old from a young 

 fish. On this part of the character therefore I can 

 only say, that the specimen in question measured twelve 

 inches in length. The back is lead blue, gradually be- 

 coming lighter at the sides, the belly being of an 

 argentine brilliancy ; while the colour about the mouth 

 and throat is the same as that of the upper parts of 

 the fish. The first dorsal fin is angular on the margin, 

 and is not above a third part of the second in length ; 

 this being rounded at the angle and continuous with 

 the caudal fin. There are no spots about the mouth. 

 The iris is white. The spiracles are of an oval shape, 

 and are nearer to each other than in the P. fluviatilis. 

 The mouth is furnished with numerous teeth, which, 

 as in the P. marinus, differ in size, the largest being 

 situated beneath the opening of the throat. They are 

 of a yellowish white colour, the palate, like the back 

 of the animal, being of a lead blue. They are disposed 

 in two irregularly circular rows, with an additional portion 

 of a row on the upper part of the palate, besides a 

 single tooth immediately above the opening of the oeso- 

 phagus. Immediately below the same opening, are three 

 bony and blunt bodies, the middle one being the largest, 

 and below these a long semilunar one ; all of them being 

 withdrawn into the throat when the circle of the mouth 

 is not in action. 



On comparing the characters now enumerated, with 

 those of the only other species with which it could be 

 confounded in description, it will be found to differ 



