Ancestry of Flounders.—The ancestry of the flounders is wholly uncertain. Because, like the codfishes, the flounders lack all fin-spines, they have been placed by some authors after the Anacanthini, or codfishes, and a common descent has been assumed. Some writers declare that the flounder is only a codfish with distorted cranium.
Fig. 429.
Figs. 429 and 430.—Larval stages of Platophrys podas, a flounder of the Mediterranean, showing the migration of the eye. (After Emery.)
A little study of the osteology of the flounder shows that this supposition is without foundation. The flounders have thoracic ventrals, not jugular as in the cod. The tail is homocercal, ending in a large hypural plate, never isocercal, except in degraded soles, in which it is rather leptocercal. The shoulder-girdle, with its perforate hypercoracoid, has the normal perch-like form. The ventral fins have about six rays, as in the perch, although the first ray is never spinous. Pseudobranchiæ are developed, these structures being obsolete in the codfishes. The gills and pharyngeals are essentially as in the perch.
It is fairly certain that the Heterosomata have diverged from the early spiny-rayed forms, Zeoidei, Berycoidei, or Scombroidei of the Jurassic or Cretaceous, and that their origin is prior to the development of the great perch stock.
If one were to guess at the nearest relationships of the group, it would be to regard them as allies of the deep-bodied mackerel-like forms, as the Stromateidæ, or perhaps with extinct Berycoid forms, as Platycormus, having the ventral fins wider than in the mackerel. Still more plausible is the recent suggestion of Dr. Boulenger that the extinct genus Amphistium resembles the primitive flounder. But there is little direct proof of such relation, and the resemblance of larval flounders to the ribbon-fishes may have equal significance. But the ribbon-fishes themselves may be degenerate Scombroids. In any case both ribbon-fishes and flounders find their nearest living relatives among the Berycoidei or Zeoidei, and have no affinity whatever with the isocercal codfish or with other members of the group called Anacanthini.
Fig. 431.—Platophrys lunatus (Linnæus), the Peacock Flounder. Family Pleuronectidæ. Cuba. (From nature by Mrs. H. C. Nash.)