30 bulletin: museum of comparative zoology. 



g. Discussion of Pfeffer's Work. 



I have purposely omitted, up to this point, any comparisons with 

 Pfetfer's work. He is the only author I have found who deals with the 

 twisting in the larval Pleuroncctidoe from other than the external point 

 of view. Unfortunately, he does not give the name of tlie species on 

 wiiich his statements are based, nor are his papers illustrated. 



In his earlier article ('86, p. 4) he describes the general conditions to 

 be found in very young Pleuronectidae. The general topography is 

 that of other young lish. The eye sockets — separated below by the 

 sphenoid [trabeculse cranii ?J, above by the " Zwischenaugen-Decke " — 

 communicate freely with each other in the intervening region. In the 

 interorbital and ethmoid regions there is a vertical ridge-like dermal 

 bone, having in cross-section the form of an elongated triangle, and sup- 

 porting the dorsal fin, which, in Pfeffer's specimens, reaches to the eth- 

 moid. This bone is still free from the cranium, and is the frontale 

 principale of authors. 



The bulbils olfactorius, which at first is lodged in the " Zwischen- 

 augen-Decke," becomes crowded backward into the brain capsule. The 

 " Interorbital-Decke " [supraorbital bar X] is bent out toward the eye 

 side and twisted somewhat on its long axis, so that its transverse axis, 

 previously horizontal, now becomes oblique, slanting downward and out- 

 ward toward the ocular side, while the chief part, which was vertical, is 

 mostly resorbed by the migrating eye. As a consequence there now re- 

 mains between the migrating eye and the surface of the head on the 

 ocular side only the thin, glass-like, scarcely perceptible outer skin 

 which previously covered the dermal bones. At the same time the der- 

 mal bone known as the froutale principale has grown fast to the inter- 

 orbital roof-piece, and its course, at first straight from the median crest 

 of the brain capsule to the ethmoid, now makes a great bend. Onlj' its 

 basal part, in the form of a broad band remains, while the vertical (and 

 at first the larger) part has been resorbed. The upper part of the wing 

 of the etlmioid on the ocular side has fused with the fronto-orbital, and 

 the upper part of its outer margin is continuous with the now develop- 

 ing supraorbital cartilage or bone, while the wing of the eyeless side 

 remains free on all sides, not forming any.connection with the supra- 

 orbital of its own side. 



This description of the relations of the wings of the ethmoid to the 

 supraorbitals resembles the condition which I have found in Stage 

 III a of P. americanus (Figure B, pp. 19, 20) ; but in P. americanusand 



