248 PISCES. 



ratus, narrow, digitiform, cartilaginous plates, the analogues of the 

 branchial rays, and which are fastened to the quadratal cartilage. 

 The peculiar disposition of the branchiae themselves in the Sharks, 

 Rays, and Cyclostomi, renders a true operculum unnecessary. 



In the majority of Fishes, but particularly in those of the Osseous 

 kind, a double row of pointed lanceolate leaflets project, like the 

 teeth of a comb, from the convex side of each of the four branchial 

 arches ; they are mostly separate as far as their base, where they 

 coalesce, but are sometimes united higher up ; each leaflet is pro- 

 vided in the middle with a thin fibro-cartilaginous plate, that keeps 

 it stiff and straight. Upon these leaflets we find a number of 

 thin membranous transverse ridges, which contribute to increase 

 the respiratory surface, and the plexuses of blood-vessels further 

 expand upon peculiar siliquose elevations ; it is rare for only three 

 of the branchial arches to support such pectiniform leaflets, as in 

 Lophius, Batrachus, Diodon, Tetrodon ; and very rarely do three, 

 or only one row of branchial leaflets rest upon an arch. The 

 branchial leaflets in Syngnathus and allied genera forming the 

 group of Lophobranchii are of an unusual form, being lanceolate, 

 but very broad and short, so as to form tufts. All these branchial 

 combs are lodged in a common cavity situated behind the oper- 

 cular apparatus, and communicating with the mouth by the slits 

 between the branchial arches, and externally by a single large or 

 frequently very small slit between the edge of the operculum and 

 the girdle of the pectoral fins. The arrangement of these parts is 

 somewhat different in the true Cartilaginous Fishes ; upon the mid- 

 dle of each of their branchial arches we find a dense cellular plate, 

 which attaches them to the external integument ; in front and pos- 

 teriorly the mucous membrane of the mouth is prolonged over this 

 plate, and forms upon it elevated folds standing perpendicularly upon 

 the cartilaginous arches, like the branchial leaflets of the Acantho- 

 pterygians ; externally the mucous membrane is continuous with the 

 external integument ; each branchial arch is furnished with an an- 

 terior and posterior row of such branchial folds, but the anterior arch 

 has only the posterior set, so that only four and a half gills are to be 

 counted. From the branchiae being united externally to the integu- 

 ment, five (and in other genera, as Hexanchus, Heptanchus, six to 

 seven) branchial fissures are found both internally and externally, 

 between which the integument forms narrow bridge-like strips. The 

 Cyclostomi present a still more peculiar structure. There are here 

 from six to seven pairs of branchiae present ; each pair forms a flat 



