These fishes represent in our fresh waters the great sub-order Ostariophysi^ soft-rayed Teleosteans, with the mesocoracoid arch present as in the Salmonids, with the ventral fins inserted far behind the pectorals, and with the air-bladder communicating with the digestive tract, and connected with the ear through a chain of bones, the so-called Weberian ossicles, attached to the strongly modified anterior vertebrae.
The Cyprinidas, almost exclusively confined to fresh waters, and represented by about 1300 species, contain most of our coarse fishes, and although the flesh of not one of them is held in very high esteem, afford pleasure to a multitude of anglers. They are easily recognised by their more or less protractile, toothless mouth, their naked head, their pectoral fins inserted very low down the sides, and folding like the ventrals, which are placed much farther back, and the absence of an adipose dorsal fin. The absence of teeth in the jaws is compensated by their great development in the throat, on the lower pharyngeal bones, on which they are arranged in one, two, or three series, and vary singularly in shape according to the genera and species. These teeth, which may be molariform, spoon-shaped, knife-shaped, entire, or serrated, work against a horny plate attached to a process of the back of the skull, extending under the anterior vertebrae. The shape and disposition of these pharyngeal teeth afford important characters for discriminating between species, but owing to the difficulty of examining them without cutting into the fish or extracting the bones which bear them, we have abstained from alluding to them in the following key, which has been specially prepared with the object of enabling anglers and others not familiar with the methods of the ichthyological system, to correctly and readily name any fish of the thirteen species occurring in British waters, leaving aside the consideration of hybrids, to which allusion is made further on.
I. Anal fin with the base shorter, or not much longer than that of the dorsal, and with not more than 12 branched rays.1
A. Dorsal fin with the base more than twice as long as that of the anal, with 14 to 22 branched rays; anal with 5 or 6 branched rays; scales large.
Mouth with two barbels on each side; 34 to 40 tube-bearing scales in the lateral line.
No barbels; 28 to 35 scales in the lateral line.
B. Dorsal fin with the base not twice as long as that of the anal, with 6 to 18 branched rays. a. Scales moderate or large, not more than 65 in the lateral line, (i.) Mouth with barbels; anal fin with 5 or 6 branched rays; dorsal fin opposite to the ventrals.
Two barbels on each side of the mouth; last simple ray of dorsal fin strong, bony, and serrated behind; 55 to 65 scales in the lateral line.
A single barbel on each side of the mouth; last simple ray of dorsal fin weak, flexible; 38 to 44 scales in the lateral line.
(II.) Mouth without barbels; anal fin with 7 to 12 branched rays.
1 The branched rays are preceded, as in the dorsal fin, by two or more simple rays, the last of which may be bony and serrated; the last branched ray, both in the dorsal and in the anal, is cleft to the base and often appears as two; even in the latter case it is reckoned as one, the bony base being single.
(*) Depth of body 2½ to 3½ times in total length (caudal fin not included); anal fin with 10 to 12 branched rays.
Dorsal fin with straight or indistinctly concave upper border, originating well behind the vertical of the base of the ventrals; 39 to 44 scales in the lateral line.
Dorsal fin with distinctly concave upper border, originating above or just behind the base of the ventrals; 42 to 46 scales in the lateral line.
(**) Body more elongate, its depth 3½ to 4 times in total length (caudal fin not included) ; anal fin with 7 to 9 branched rays.
Anal fin with convex border; dorsal fin originating just behind vertical of base of ventrals; 42 to 50 scales in the lateral line, 3 or 3½ between lateral line and ventral fin.
Anal fin with straight or slightly concave border; dorsal fin originating above base of ventrals; 47 to 52 scales in the lateral line, 4 or 5 between lateral line and ventral fin.
b. Scales very small, 80 or more in the lateral line; anal fin with 6 to 8 branched rays.
No barbels; caudal fin strongly emarginate.
A minute barbel on each side of the mouth; caudal fin not, or but slightly, emarginate.
II. Anal fin with the base much longer than that of the dorsal, with 16 or more branched rays; dorsal fin opposite to the space between the ventral and anal fins.
A. Body deep, its depth 2 to 3 times in total length (caudal fin not included).
Anal fin with 23 to 30 branched rays; 50 to 57 scales in the lateral line, 11 to 13 between the dorsal fin and the lateral line.
Anal fin with 17 to 23 branched rays; 40 to 50 scales in the lateral line, 9 or 10 between the dorsal fin and the lateral line.
B. Body more elongate, its depth about 4 times in total length (caudal fin not included).
Anal fin with 16 to 20 branched rays; 47 to 55 scales in the lateral line, 8 or 9 between the dorsal fin and the lateral line.
In addition to the forms here defined, aberrant individuals occasionally occur, which have been ascertained to be hybrids between distinct species, sometimes even of different genera, the distinctive characters of which they combine. Thus the following crosses are known to have occurred in ponds in this country :-