Fig. 418.—Outlines of various Pleuronectids, showing differences of form. A, Psettodes erumei; B, Pleuronectes platessa; C, Psetta maxima; D, Solea vulgaris; E, Cynoglossus lingua.
A satisfactory classification of the Pleuronectidae is still a desideratum, and cannot be attempted until the osteology of the very numerous forms has been thoroughly studied. Even the division into two principal groups, regarded by some recent authors as families, Pleuronectidae and Soleidae, is based on characters which the examination of a large number of generic types shows not to be constant. Thus the former have been defined as having the praeopercular margin distinct externally, the snout not projecting beyond the mouth, the nostrils of the two sides on the coloured side or those of the blind side high up near the dorsal line of the head; the latter as having the praeoperculum hidden under the skin, the snout projecting more or less beyond the mouth, and the nasal organ of the blind side similarly situated to that of the eyed side. However, the genera Aphoristia and Peltorhamphus, and others among the Soles, show exceptions to this definition.[[729]]
Principal genera: Psettodes, Atheresthes, Platysomatichthys, Hippoglossus, Hippoglossoides, Hippoglossina, Poecilopsetta, Chascanopsetta, Paralichthys, Pleuronectes, Glyptocephalus, Citharus, Rhomboidichthys, Psetta (Rhombus), Arnoglossus, Zeugopterus, Lepidorhombus, Ammotretis, Rhombosolea, Solea, Achirus, Achiropsis, Soleotalpa, Synaptura, Ammopleurops, Aphoristia, Cynoglossus, Symphurus (Plagusia).
The following are the principal British representatives which are valued as food:—The Halibut (Hippoglossus vulgaris), by far the largest of all Flat-Fishes, growing to a length of 10 feet or more; the Long Rough Dab (Hippoglossoides limandoides); the Plaice (Pleuronectes platessa); the Flounder (P. flesus), which ascends streams; the Dab (P. limanda); the Smear Dab, often called Lemon Sole (Glyptocephalus microcephalus); the Witch (G. cynoglossus); the Megrim or Whiff (Lepidorhombus megastoma); the Turbot (Psetta maxima); the Brill (P. laevis); and the Sole (Solea vulgaris).
Division IV.—KURTIFORMES.
No bony stay for the praeopercle. Dorsal spines feeble, few. Scapula absent, the coracoid supporting four small pterygials. Ventral fins thoracic.
Fam. 1. Kurtidae.—The genus Kurtus, with a single species, from the Indian and Pacific Oceans, forms an isolated, very aberrant group. The strongly compressed body is covered with minute, rudimentary scales; the dorsal is short, with few, graduated spines, and the anal much elongate, with 2 small spines; the ventrals are formed of 1 spine and 5 soft rays. The vertebral column consists of 24 vertebrae; the ribs of the third and fourth are free and slender, whilst the following are immovably fixed between rings formed by the ossification of the outer membrane of the elongate air-bladder in a manner unique among fishes. The skull is peculiar for its very strong, denticulate, occipital crest, which ends posteriorly in a curved spine bent forwards; the suborbitals are slender and do not emit a subocular lamina. Kurtus indicus does not exceed a length of 5 inches.
Fig. 419.—Skeleton of Kurtus indicus.