PARKEK: OPTIC CHIASMA IN TELEOSTS. 235 



their nearest present relatives are probably the Gadidae. The Gadidae, 

 however, have a body very differently formed from that of any living 

 flatfish, and if they were ancestral to the present flatfishes, there must 

 have been intermediate members whose bodies were flattened sidewise 

 and were probably symmetrical. A fish of such proportions is seen in 

 the modern Zeus faber. Without going the length that Thilo (:02) 

 does and assuming that this fish really represents the forerunners of the 

 flatfishes, it seems certain that the ancestors of these fishes must have 

 had much the proportions of Zeus. From fishes of such form the 

 unsymmetrical flatfishes have doubtless been derived. Their symmet- 

 rical ancestors, like all other symmetrical teleosts, probably had dimor- 

 phic chiasmata. That tliis feature was handed on to the flatfishes is 

 evident from the fact that it still characterizes the whole family of soles. 

 I am aware that the soles are usually regarded as degraded Pleuronec- 

 tidae, and they certainly are in many respects degenerate ; but, from the 

 standpoint of their chiasmata, they certainly present the most primitive 

 conditions seen in any flatfish, and I believe, therefore, that they are 

 degenerate descendants of the original stocli of flatfishes that had not 

 yet passed beyond the stage of dimorphic chiasmata. From this stock 

 was differentiated the Pleuronectitlae by a process whereby, amongst 

 other things, a monomorphic chiasma was produced. This type of 

 chiasma was differentiated in two lines so as to meet the requirements, 



(1) of a sinistral type of symmetry, as in the Psettinae, or turbots, and 



(2) of a dextral type, as in the Pleuronectinae, or flounders proper. In 

 the tribes thus established species here and there varied in their sym- 

 metry as in the starry flounder, etc., but in such instances the char- 

 acter of the chiasma indicates at once whether the species belongs to 

 a stock originally sinistral or dextral. Such changes as these must be 

 looked upon as the most recent realized by the flatfishes. 



It would be a matter of great satisfaction if the ancestry of the flat- 

 fishes could be traced through their fossil remains. Unfortunately the 

 scantiness of such material renders this impossible, though the occurrence 

 of a Rhombus in the upper eocene and of a Solea in the miocene points 

 to the antiquity of these fishes among teleosts. 



Throughout the whole of the preceding discussion on the Pleuronec- 

 tidae, it has been assumed that the dorsal position of the nerve con- 

 nected with the migrating eye is a real advantage to the animals 

 possessing it. In fact, the explanation of the prevalence of the mono- 

 morphic condition in the Pleuronectidae rests upon this assumption. It 

 is by no means easy to show that this assumption is, as I believe it to be, 



VOL. XL. — NO. 5 2 



