4 86 



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- 

 tebrae 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 40 (10 +30). The apparent signifi- 



FIG. 432. FIG. 433. 



FIG. 432. Heterocercal tail of young Trout, Salmo fario Linnaeus. (After Parker 



& Haswell.) 

 FIG. 433. Homocercal tail of a Flounder, Paralichthys californicus. 



cance 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 vertebrae. 



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 



