The location of the fins of the fish mark the subdivisions of the families. The above diagram being supposed to represent a fish, and a Trout at that, G is the first back or dorsal fin, F the second--in the case of this species, mere rayless, fatty matter; E is the tail fin or caudal--the writer, as a married man, naturally avoiding the latter term on account of its suggestiveness; D is the anal fin, for which the writer can offer no English substitute; C are the two ventrals or belly fins; B is the pectoral or shoulder fin, having a complemental one on the other side of the fish; and A represents what in learned language are called _branchiostegous_ rays, a name that, being translated, means merely gill-rays. What is not in a name! H is the lateral line. Then bearing in mind the great divisions of soft and hard finned, the subdivisions are distinguished by the fish having the ventrals behind the pectorals and on the abdomen, giving them the name of _abdominal_ fish, or before the pectorals, giving rise to the name _jugular_ or throat finned, and below the pectorals, giving the name _thoracic_ or shoulder-finned fish. Philosophers pay little attention to the dorsal and anal fins, and fish, without losing their identity, can have as many as they please. In caudals, unlike human Caudles, they are restricted to one. There are other fish, such as eels, denominated _apodal_ or _footless_, because the lower fins or feet are wholly wanting.
After having examined the texture, number and location of the fins, and counted the number of the rays in each, the naturalist next turns his attention to the hard bony portion of the head, which covers the gills, and opens and shuts as the fish breathes, and which, with the excellent common sense for which naturalists are notorious, he calls the _operculum_. It is divided into the _operculum_, or gill-cover proper, No. 1; the _pre-operculum_, or fore gill-cover, No. 2; the _inter-operculum_, or middle gill-cover, No. 3; and the _sub-operculum_, or under gill-cover, No. 4. The head, in the foregoing diagram, is intended to represent the head of a trout, weighing a pound and a half, caught at Phillipse’s Pond, near Smith Town, Long Island. The gill-rays are shown at No. 5. The divisions of the gill-cover are faintly marked in the real fish, and require some study.