Jordan and Evermann. Fishes of North America. 2603 



nearly 500. The group "Bibroniidce" recently recognized by some of the 

 Italian ichthyologists as a separate family ("Bibronidi"}, is composed 

 entirely of larval forms in the early stages of their development. In this 

 condition the eyes are symmetrical and the body translucent. Several 

 generic names have been given to these peculiar forms (Peloria, Bibronia, 

 Coccolus, Charybdia, Bascanius, Delothyris), but, of course, these genera 

 can have no permanent place in the system. Peloria has been shown by 

 Dr. Emery to be the young of Platophrys. The others seem to belong to 

 the Cynoglossince or to some allied group, but we are not yet certain as to 

 the correct identification of any of them. We recognize among the Pleuro- 

 nectidcv 6 subfamilies Hippoglossince, Psettince, Samarince, Pleuronectince, 

 Oncopterincc, and Pelecanichthyincc. These subfamilies are natural groups 

 and are in most cases easily distinguished, although some few aberrant 

 genera exist, which serve as links joining one group to another. Thus 

 Isopsetta of the Pleuronectince is certainly a near ally of Psettichthys, which 

 is as certainly a genuine member of the Hippoglossince. The Hippoglossince 

 and the Pleuronectince are largely arctic in their distribution, few of the 

 former group and none of the latter extending into the Tropics. The 

 Oncopterinm seem to take the place of the Pleuronectince in antarctic waters, 

 but the species of this group are few in number. The Psettince, and the 

 soles are, on the other hand, essentially warm- water fishes, their represent- 

 atives in the north being comparatively few. The Samarince are few in 

 number and belong to the East Indian fauna, and the single species of 

 Pelecaniclitliyince belongs to the bassalian fauna of the Pacific. As the 

 tropical Hippoglossince and all the Psettince are sinistral species, the eyes 

 and color being on the left side of the body, it follows that the tropical 

 flounders are nearly all left-sided species, while those of arctic and ant- 

 arctic waters are chiefly dextral species, the eyes and color on the right. 

 The Hippoglossince are the most generalized of the flatfishes. From the 

 northern representatives of this group, the allies of Hippoglossoides, the 

 Pleuronectince, are certainly descended. The Psettince are apparently 

 derived from ancestors of the type of Paraliclithys. The soles show closest 

 affinities with the Psettincc, from ancestors of which group they have 

 become degraded. Very remarkable is the relation between the number 

 of vertebrae and the geographical distribution of the various species. It 

 has been already noticed by Dr. Gill, Dr. Giinther and others that in some 

 groups of fishes northern representatives have the number of vertebra? 

 increased. In no group is this more striking than in the flounders, as the 

 following table, showing the numbers of the vertebrae in various species, 

 will clearly show. The numbers inclosed in brackets are copied from Dr. 

 GUnther; the others represent our own count of specimens. 



Numbers of vertebrae in flounders. 

 I. HIPPOGLOSSIN.E. 



Hippoglossus hippoglossus 16 + 34 = 50 



Atheresthes stomias 12 + 37= 49 



Hippoglossoides platessoides 13 + 32 = 45 



Lyopsetta exilis 11 + 34 == 45 



Eopsetta Jordan! 11+32= 43 



