Notes on the British Species 0/ Zeugopterus. 3G5 



Distrihittion. — Sliores of Northern Europe to tlie Mediter- 

 ranean. 



A feature common to all the species of the genus Zeugoptcrus, 

 as here understood, is the constant occurrence of a flap-like 

 accessory portion of the vertical fins situated under the tail. 

 In a former paper (10) I described the use of this peculiar 

 modification of the vertical fins in aiding respiration. From 

 further observation of its action, I am inclined to think too 

 much stress was then laid on the action of the accessory 

 flap. Under normal circumstances, when a specimen of 

 Zeugopterus adheres closely to a smooth surface by a back- 

 ward pressure of the rays of the dorsal and anal, the bod}^ is 

 arched, and a space is left between the under side of the fish 

 and the surface to which it adheres. Through this channel 

 a constant current of water is made to pass from the branchial 

 chamber on the under side to the tail. The basal portions of 

 the vertical fins are kept in constant motion during this 

 time, but the motion is more vigorous in the rays imme- 

 diately in front of the tail than in the accessory flaps situated 

 ■underneath it. 



In Z. punctcttus and Z. papillosus the ventrals are united 

 with the anal, the anal papilla being situated between the 

 two ventrals. In Z. unimaculatus, on the contrary, the 

 ventrals are free. The anal papilla is, however, situated 

 between the two ventrals as in the other species, and the 

 rays of the anal fin near its anterior extremity become shorter 

 and shorter, and the fin gradually tapers away to the anal 

 papilla. The ventral fins, therefore, cover the anterior ex- 

 tremity of the anal on both sides, and thus appear to be 

 continuous with it in a living specimen. It is only when the 

 ventrals are pulled to one side that they are found to be 

 unattached to the anal. The position of the anal papilla 

 between the ventrals, and not behind them as in other Pleuro- 

 nectidce, appears to be of generic value. 



The ciliated cycloid scales are also characteristic, and 

 give the species of this genus an appearance peculiar to 

 themselves amongst British Pleuronectidse at least. Figures 

 of these scales will be found on Plates XIV.-XVI. 



VOL. IX. 2 B 



