On the natural history and classification of fishes, amphibians, and reptiles / by William Swainson.

  • William Swainson
Date:
1838-1839
Catalogue details

Licence: Public Domain Mark

Credit: On the natural history and classification of fishes, amphibians, and reptiles / by William Swainson. Source: Wellcome Collection.

    45/386 (page 33)
    the external ones, so that they form a lobe on each of their sides — one above, the other below. Where such numerous gradations occur, it is impossible to define with strictness the limits of our definitions; we may, never- theless, arrive at some degree of precision, by con- sidering each of these forms as presenting the following modifications : — A rounded fin is either lanceolate, oval round, even, or truncate. On reaching this latter, we may draw an imaginary line, and enter upon the fork- tailed division. Truncate fins pass into those which are slightly crescent-shaped; they next become lunate, forked, lobed, and, finally, emarginate; while these latter, again, pass into rounded fins: the highest development of each of these is seen in the lanceolate and the forked ; the other modifications gradually recede from each other. (35.) We shall now endeavour to define each of these forms.—1. Lanceolate, named from the shape bearing a resemblance to the head of a lance: the longest ray is in the centre, and stands singly; all the others are in pairs, diminishing, more or less gradually, in length, until those that are external become the shortest. Nearly all the species of Cepola {fig. 3. a) possess this character, but it is by no means frequent, and is chiefiy seen in that genus, Gobius, and in Scicena pama (Cuv. pi. 101.). — 2. Oblong oval: not quite so long in pro- portion as the last; the middle is not pointed, and the shape is that of the smaller end of an egg. This form may be called a highly developed state of the next, and is confined to few examples. — 3. Round : the fin is of moderate size, always shorter than the last, and the extremity describes the segment of a circle. This is the most common shape in this division, and pervades all the flat fish {PleuronectidcB), a few of the rocklings {Motella Cuv.), all the gobies {Gobiadm), the genus Syngnathus, Sic., none of which are capable of long sustained swimming: the degree of roundness varies; but we still retain the name to all such fins as have the central rays in any degree longer than those on their sides. — 4. Even : the majority of the rays are VOL. I. D