Suborder Heterosomata 697 
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. 
Soles have been found in the later Tertiary rocks. Solea 
kirchbergiana of the Miocene is not very different from species 
Fic. 602—Window-pane, Lophopsetta maculata. Virginia. 
now extant in southern Europe. No remains referable to 
allies of the halibut or plaice are found in Tertiary rocks, and 
these relatively simple types must be regarded as of recent origin. 
The Turbot Tribe: Bothine.—The turbot tribe have the mouth 
large, the eyes and color on the left side, and the ventral 
fins unlike, that of the left side being extended along the ridge 
of the abdomen. The species are found in the warm seas only. 
They are deeper in body than the halibut and plaice, and some 
of them are the smallest of all flounders. It is probable that 
these approach most nearly of existing flounders to the original 
ancestors of the group. 
Perhaps the most primitive genus is Bothus, species of 
which genus are found in Italian Miocene. The European 
