288 SUMMARY OF CURRENT RESEARCHES RELATING TO 



their dorsal nerves connected with their migrating eyes. In all species 

 that have both dextral and sinistral individuals, the dorsal nerve is con- 

 nected with that eye which in the greatest number or in the nearest of 

 kin migrates. 



The unmetamorphosed young of the Pleuronectida? are not sym- 

 metrical in the same sense that symmetrical Teleosts are, for they have 

 monomorphic chiasmata. The Soleida? are not degraded Pleuronectidae, 

 but degenerate descendants of primitive flat-fishes, from which the 

 Pleuronectidae have probably been derived. The monomorphic condi- 

 tion of the optic chiasma of the Pleuronectidae can be explained only 

 on the assumption of natural selection. The flat-fishes afford striking 

 examples of discontinuous variation. 



Lateral Canals and Cranial Bones of Polyodon folium.* — E. Ph. 

 Allis, jr., discusses these, and proposes certain homologies which show, 

 if correct, that while Polyodon more closely approaches Selachians in 

 the arrangement of its lateral canals than any known Teleost or other 

 Ganoid, the bones enclosing these canals more closely approach the 

 arrangement found in Amphibians than those of any other known 

 living fish, excepting Glorias, which, according to Huxley, closely 

 approaches Coccosteus in its dermal armature. 



The conditions found in Polyodon, moreover, definitely establish the 

 fact that there is a definite lateral canal component in certain of the 

 so-called dermal bones of the skull of fishes, and that this component 

 may be found wholly separate and distinct from another, so-called mem- 

 branous component that may form part of the same bones. Is then 

 this lateral component retained after the sensory organ or organs in 

 relation to which it has developed have disappeared ? That it may be 

 retained without a related enclosed organ is certainly shown by the 

 presence, in Conger, of a postauditory squamosal canal ossicle without 

 such a related organ. 



Flight of Flying Fish.f — G. E. H. Barrett-Hamilton reports his 

 observations on this much discussed subject. While some naturalists, 

 such as Whitman, maintain stoutly that the fins of Exocoztus are flapped, 

 others, such as Mobius, deny that genuine muscular movements ever 

 occur. The author corroborates Moseley's observation that in Exomtas 

 at all events the wings are never moved as organs of true flight. They 

 may vibrate or quiver under the action of air currents or the shifting a 

 little of their inclination by the fish, but the whole motive power is 

 supplied by the powerful tail. The wings are a parachute to augment 

 the action of this propeller, but their motions are in no way comparable 

 to those of the wings of a bird. It is suggested that some of the dis- 

 crepancies in the descriptions of well qualified observers may be explained 

 by supposing that different fishes were studied. There is really almost 

 complete accord on the one important point, namely, that the so-called 

 " fluttering " or " flapping," if it does occur, is almost invariably dis- 

 continued after the flight has really commenced. It is in nearly every 

 case merely an accompaniment of the initial spring into the air. 



* Zool. Jahrb., xvii. (1903) pp. 659-78 (1 pi. and 2 figs.). 

 t Ann. Nat. Hist., xi. (1903; pp. 389-93. 



