Evolution of the Order Heterosomata. 487 



below ; the pelvic fin, preserved only in the Monte Bolca 

 specimen, is formed of a spine and, in my opinion, 5 soft rays, 

 for I cannot see a greater number inserted on the pelvic bone 

 which lies uppermost, the outlines of which are fairly distinct. 



Boulenger's restoration shows several features of Psettodes 

 or Zeus rather than Psettus which I am unable to see in the 

 fossils ; thus he shows the lower jaw nearly as long as the 

 head and the praeoperculum vertical and scarcely curved, 

 whereas the lower jaw appears to me only a little more than 

 half the length of the head, and the prasoperculum to have a 

 distinct lower limb ; also the origin of the anal fin is not so 

 far forward in the actual fossils as it is in the restoration. 



Bo thus and Solea were already in existence in the Upper 

 Eocene, and, indeed, the whole Upper Eocene fish-fauna is 

 strikingly modern, so that there is no reason to regard 

 Amphistium as ancestral to the flat-fishes on account of its 

 occurrence in the Upper Eocene. 



The researches of Parker * on the optic chiasma are of 

 great importance for the classification of the Heterosomata. 

 lie found that in various symmetrical Teleosts the left nerve 

 crossed above the right about as frequently as the right above 

 the left; this was also the case in flat-fishes of the family 

 ►Soleidse as recognized by Jordan and Evermann f, whether 

 dextral (Sotea, Achirm) or sinistral (Symphurus). From 

 this dimorphism of the chiasma it follows that in the Soleidae 

 the optic nerves are partly uncrossed when the nerve of the 

 migrating eye is dorsal, and that they almost cross each other 

 twice when it is ventral. In other flat-fishes, whether dextral 

 (Pxettichthys, Atheresthes, Parcphrys, Pleuronectes, &c.) or 

 sinistral (Paralichthys, Platophrys, Citharichthys,&c.) , Parker 

 found that it was always the case that the nerve of the 

 migrating eye was dorsal, the only exception being in the 

 case of reversed examples, i. e. sinistral members of dextral 

 species or dextral members of sinistral species, in which that 

 nerve was dorsal which was normally dorsal in the genus. 

 In a few species of the Pacific coast of North America sinistral 

 and dextral individuals are equally numerous ; but in a species 

 of a sinistral genus, such as Paralichthys californicus, the 

 nerve of the right eye is always dorsal, whether the individual 

 be sinistral or dextral ; similarly, in a species of a dextral 

 genus, such as Platichthys stellatus, the nerve of the left eye 

 is dorsal. This monomorphism of the optic chiasma is 

 obviously a specialization, which Parker considers has been 



* Bull. Mus. Comp. Zool. xl. pp. 219-242 (1903). 



t Bull. U.S. Nat. Mus. xlvii. pt. iii. pp. 2602-2712 (1898). 



