382 FISHES. CYCLOSTOMI. 



SUB-CLASS I. CAR TIL A GINO US FISHES 



(Chondropterygii, Cuv.) 



THE bones of tliis division are essentially cartilaginous, and in general are never 

 formed of bony fibres. The calcareous matter being deposited in minute grains, and 

 not in fibres or filaments, it necessarily follows that no sutures occur in the cranium, 

 which is always in a single piece. The parts analogous to those in the crania of 

 other fishes may however be distinguished by projections, hollows, and openings. 

 Even the moveable articulations of the other division are not found in this ; for in some 

 of the rays a part, and in the lampreys the whole, of the vertebral column is united, 

 and the divisions not distinguished but by annular marks. In the greater number 

 of the genera also, some of the articulated portions of the bones of the face are want- 

 ing. In other respects however, as for instance in the nervous system, and in the 

 organs connected with nutrition, the arrangement of parts is as complete as in the 

 other fishes ; and many genera have besides the organs of reproduction in a form 

 equally perfect with those of the Reptiles. 



The most general character common to the Cartilaginous Fishes, and sufficient to 

 distinguish them from all others, is that of wanting entirely, or having only in a rudi- 

 mentary form, the maxillary and intermaxillary bones, the place of which is supplied 

 by bones analogous to the palatine ones, and even sometimes by the vomer. 



This sub-class Cuvier divides into three Orders., viz. those which have the branchiae 

 fixed, which includes the Cyclostomi and Plagiostomi ; and those in which the bran- 

 chiae are free, which comprises the Sturiones. 



ORDER I. CYCLOSTOMI. 



Jaws fixed in an immoveable ring; branchiae fixed, and the 

 openings numerous. 



The branchiae in this order, in place of being free at their external margin, and 

 all the intervals opening into a common receptacle, as is generally the case, are on 

 the contrary attached to the skin ; and the water escapes by as many openings in 

 this skin as there are intervals between them. Another peculiarity in these fishes 

 consists in having small cartilaginous arches or ribs suspended in the flesh at the ex- 

 terior margin of the branchiae. 



The fishes of this order are with regard to their skeleton the most imperfect of 

 vertebrated animals. Destitute of pectoral and ventral fins, their elongated body 

 is terminated before by a circular fleshy lip, supported by a cartilaginous ring. The 

 vertebrae are united in a tendinous cord, filled interiorly with a mucilaginous sub- 

 stance, and covered exteriorly with indistinct cartilaginous rings, more or less solid. 

 The ordinary ribs are wanting, but the branchial ribs or supports, scarcely visible 

 in the sharks and rays, are much developed, and united to one another. The bran- 

 chiae in place of being pectinated, as in all the other fishes, have the appearance of 

 purses, from the junction of their faces to the opposite faces of the neighbouring 

 ones. The labyrinth of the ear is inclosed in the cranium ; the nostrils open by one 

 orifice, before which is a blind cavity, which some writers have improperly named 

 a spiracle ; and the intestinal canal .is short. 



Gen. 1. PETIIOMYZON, Dum. Lin. 



Maxillary ring armed with strong teeth ; lips formed for suc- 

 tion ; tongue with two rows of small teeth ; a dorsal fin before 

 and another behind the anus. 



P. marinus, Lin. The greater, or Sea Lamprey. Body yellowish, 

 marbled with brown ; silvery below ; the first dorsal fin distinct 

 from the second ; two large approximated teeth at the top of the 



