696 Suborder Heterosomata 
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 ver- 
tebree and of fin-rays. The halibut has 50 vertebrae (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 4o (10 +30). 
Fia. 601. 
Fic. 600.—Heterocercal tail of young Trout, Salmo fario Linmeus. (After Parker 
& Haswell.) 
Fig. (01.—Homocercal tail of a Flounder, Paralichthys californicus. 
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 vertebra. 
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 
