Evolution of the Order Heterosomata. 



489 



quite appreciating their full importance. Adopting Jordan 

 and Evermann's two families, Pleuronectidse and Soleidae, he 

 has found that in the former the nasal organ of the blind side 

 has accompanied or followed the eye in its migration, and 

 lies nearly on the edge of the head, whereas in the latter the 

 nasal organs are placed symmetrically ; this confirms Parker's 

 ideas as to the Soleida?, and I may mention that, although in 

 Psettodes the nasal organs are not quite symmetrical, they are 

 very nearly so. 



Kyle has also found that in Psettodes, the sinistral Pleuro- 

 nectoids, and the Soleidse the olfactory laminae radiate from 



Fig. 3. 



/7 



~-,ol 



B. 



Nasal organ of eyed side of (A) Halibut (Hippoglossus hijjpoglossus) and 

 (B) Turbot (Bothus tnaximus). (After Kyle.) 



ol, olfactory laminae ; n$, nasal sacs ; n, nasal bones. 



or are arranged transversely to a median rachis, as in most 

 Teleosts, whereas in the dextral Pleuronectoids (except the 

 Soleidge) the laminge are parallel and there is no rachis. 

 Unfortunately Kyle's researches preceded Parker's, which 

 would perhaps have given him the idea of the primary im- 

 portance of sinistrality and dextrality, in spite of reversed 

 examples, and might have led him to utilize the differences 

 in the structure of the pelvic fins for the definition of sub- 

 ordinate groups only. 



The distinctness of the sinistral and dextral Pleuronecti- 

 formes from each other is shown not only by the structure 

 of the olfactory organs and the monomorphism of the optic 

 chiasma, but also by the eggs. Those of Hippoglossus, Hip- 

 poglossoides, Pleuronecles, Microstomas, and Glyptocephalus 

 are known to have an undivided yolk without oil-globule ; 

 whilst those of Paralichthys, Citharus, Platophrys, Arno- 

 glossus, Bothus, Lepidorhombus, Phrynorhombus , and Zeugo- 

 pterus are distinguished by the presence of a single oil-globule 

 in the yolk. The egg of Solea differs again, having a number 

 of small oil-globules at the surface of the yolk. 

 Ann. & Mag. N. Hist. Ser. 8. Vol. vi. 33 



