The Heterosomata are found in all seas, always close to the bottom and swimming with a swift, undulatory motion. They are usually placed in a single family, but the degraded types known as soles may be regarded as forming a second family.
The Flounders: Pleuronectidæ.—In the flounders, or Pleuronectidæ, the membrane-bones of the head are distinct, the eyes large and well separated, the mouth not greatly contracted, and the jaws always provided with teeth. Among the 500 species of flounders is found the greatest variation in size, ranging in weight from an ounce to 500 pounds. The species found in arctic regions are most degenerate and these have the largest number of vertebræ and of fin-rays. The halibut has 50 vertebræ (16 + 34), the craig-flounder 58, while in Etropus and other tropical forms the number is but 34 (10 + 24). The common flounders of intermediate geographical range (Paralichthys dentatus, etc.) show intermediate numbers as 40 (10 + 30). The apparent significance of this peculiar series of fact is given on page 212, Vol. I. It is, perhaps, related to the greater pressure of natural selection in the tropics, showing itself in the better differentiation of the bones and consequently smaller number of the vertebræ.
Fig. 432.—Heterocercal tail of young Trout, Salmo fario Linnæus. (After Parker & Haswell.)
Fig. 433.—Homocercal tail of a Flounder, Paralichthys californicus.]
Fossil flounders are very few and give no clue as to the origin of the group. In the Eocene and Miocene are remains which have been referred to Bothus (Rhombus). Bothus minimus is the oldest species known, described by Agassiz from the Eocene of Monte Bolca. In the Miocene are numerous other species of Bothus, as also tubercles referable to Scophthalmus.
On the testimony of fossils alone the genus Bothus, or one of its allies, would be the most primitive of the group. If it be so, the simpler structure of the halibut and its relatives is due to degeneration, which is probable, although their structure has the suggestion of primitive simplicity, especially in the greater approach to symmetry in the head and the symmetry in the insertion of the ventral fins.
Fig. 434.—Window-pane, Lophopsetta maculata. Virginia.