488 



Mr. 0. T. Regan on the Origin and 



arrived at on account of its mechanical advantage, and he 

 draws tlie deductions that tlie Soleidse are a natural group 

 and that thej have evolved from more generalized flat-fishes 

 than those witii a monomorphic chiasma. 



Parker did not examine Psettodes. I find that in two 

 sinistral examples of this monotypical genus the right nerve 

 is dorsal in one, ventral in the other ; this establishes that 



Fijr. 2. 



A. 



B. 



Dorsal views of anterior parts, of brains with cerebral hemispheres 

 removed, showing eyeballs, optic nerves, and optic lobes. (After 

 Parker). 



A. ParalichtJiys calif or nicus^ sinistral species, with nerve of right eye 



dorsal. 1, sinistral individual ; 2, dextral individual. 



B. Tlatichihys stellatus, dextral species, with nerve of left eye dorsal. 



3, sinistral individual ; 4, dextral individual. 



the chiasma is dimorphic, and it appears to me that in all 

 ]>robability the presence of sinistral and dextral individuals 

 in equal numbers is primitive and is of another nature from 

 the phenomena observed in some species of Paralichthys 

 and Plaiiclithys. 



Kyle"^ has written a valuable pa))er on the classification of 

 the Heterosomata, and has used some new characters without 



* Rep. Fisheries Board Scotland, xviii. 1900, pp. 335-368, pis. xi.-xil. 



