Evolution of the Order Ileterosomala. 



489 



quite appreciating tlii-ir full importance. Adoptinn^ Jordan 

 and Evcrmann's two faniilics, Pleuronectidaj and Solcidic, he 

 Jias found thai in the former the nasal organ of the hlind side 

 has accompanied or followed the eye in its migration, and 

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

 iiasal organs are placed symmetrically ; this confirms Parker's 

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

 Pstttodes 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 Soleidie the olfactory lamiice radiate from 



Fig. 3. 



-•ol 



Nasal organ of eyed side of (A) Hiilibiit {]lij)j'0(/loiisiis hippoijlossut) and 

 (bj Turbot {liothus maxinuis). (After Kyle.) 



o/, olfactory laminso ; ns, nasal sacs ; n, nasal bones. 



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

 Teleost.s, whereas in the dextral Pleuronectoids (e.\cept the 

 Soleida?) the laminje 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 j^elvic fins for the definition of sub- 

 ordinate grou|)S only. 



The distinctness of the sinistral and dextral Pleuronecti- 

 fornics from each other is shown not only by the structure 

 of the oltactory organs and the monomorphi.sm of the optic 

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

 poglossuideft, Pleiironectes, Microstoniits, and Gli/ptocephaliis 

 are known to have an undivided yi»lk without oil-globule ; 

 whilst those of Paraliclithys^ Citluirns^ Platophrys, Arno- 

 ylossus^ liot/ius, Lepidorhomhus^ Phrynorhombus, and Zeuno- 

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

 m the yolk. The egg of Solea diflTors again, having a number 

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

 Ann. cO Maj. X. Uist. Ser. 8. Vol vi. 33 



