128 CLASS XIV. 



occur also at times in fresh water. Tliey are foiincl in both, 

 hemispheres and in all climates. On account of their dentate scales 

 Agassiz refers this family to the Ctenoides, which division, but for 

 it, contains Acantho2}ter9/g{i siione ; but we must observe, that the 

 rays of the dorsal and anal fins, although soft, are still mostly not 

 split into threads. This family forms in fact the transition to the 

 AcanthojJterygii. In almost all the species there are 7 rays in the 

 gill-membrane j Pleuronectes limandoides Block has 8. 



These fishes are distinguished especially by the position of the 

 eyes on one side of the head ; in some they lie on the right side, 

 but in as great or perhaps even a greater number of species on the 

 left. Sometimes varieties are observed in this respect, and the eyes 

 lie on the opposite side to that on which they are usually placed in 

 that species, which has been often seen in Pleuronectes Jlesus L., the 

 flounder; nay, it has even been obsei-ved, that on both sides of the 

 head an eye was placed, as in other vertebrate animals'. The side 

 on which these eyes are placed is coloured, the other white; the 

 side with the eyes is in some sense to be regarded as the back, and 

 in swimming is turned upwards; often the jaws are more strongly 

 developed on the opposite side, and the teeth larger; sometimes 

 these last are entirely wanting on the upper side. On account of 

 the great development of the tail the abdominal cavity is j^laced 

 quite forward and is very small. The pectoral fins, which are of 

 small service in swimming, are wanting in some, and are always 

 little developed. The ventral fins also, which lie close together and 

 are situated under the pectoral fins, always remain very small. 



On the structure of these fishes interesting remarks may be found 

 by AuTENRiETH in Wiedemann's Archiv fur Zoologle u. Zoot. i, i8oo, 

 s. 57 — 130; skeletons are figured in Eosenthal Ichthyot. Taf. xi. 



See P. J. Van Beneden Note sur la Symmetrie cles Poissons pleuronectes 

 dans leurjeune dge, Ann. des Sc. nat. 3e S6rie, xx. pp. 340 — 342. 



Pleuronectes QuENSEL, Sundev. Maxillge apparent externally, 

 not covered with scales. Superior maxillary bone with free, dilated 

 apex. Opercula free above pectoral fins. Eyes moderate. Nostrils 

 remote from jaws, placed on the blind side at the dorsal margin. 

 Caudal fin always distinct, truncate or rounded. 



^ In Pleuronectes maxiimis; see Schleep, Oken's Isis, 1829, s. 1049— 1053, 

 Taf. III. 



