1910.] AND INVERTEBRATES OF ST. HELENA. 117 



in the smaller specimen there is no intra-marginal line and the 

 edge of the fin is straight, the corners not being prolonged. On 

 examining the generative organs I found that the smaller specimen 

 was a female and the larger a male, the oi'gans in both being large 

 and well developed. The slight differences described avouM thus 

 appear to be sexual, but in Batistes vetula, of which I brought a 

 specimen 14| ins. long from Ascension, the dorsal and ventral 

 caudal rays and also the anterior rays of the second dorsal are 

 much prolonged, and the specimen is female. It is of course not 

 unusual to find a character confined to the male in one species 

 present in both sexes in another. 



Batistes buniva is not very common at St. Helena ; I obtained 

 one specimen from the fishermen, and another was caught on a 

 ground-line off George Island on the windward side of the main 

 island. At Ascension it is very abundant ; on the outward voyage 

 when the ship anchored off Georgetown shoals of these fishes came 

 alongside to feed on pieces of orange-peel and other fragments 

 thrown overboard. I went down the boat ladder with some pieces 

 of biscuit to try to catch one, and when I had no more biscuit 

 simply moved my fingers about at the surface of the water ; 

 a number of the fishes crowded round my hand and I was able to 

 seize one and lift it into the boat. On the return voyage some 

 were caught by passengers with hook and line, and on one line 

 let down to a greater depth was caught the specimen of Batistes 

 vetula mentioned above, the bait used being pieces of raw beef. 

 Both these species have a thin patch of skin covered by angular 

 scutes instead of scales just above and behind the base of the 

 pectoral fin ; beneath this membrane is an air-cavity and the 

 whole forms a drum by which a sound is produced. I heard this 

 sound when I held a specimen of either species in the hand, and 

 noticed that when it was produced the pectoral was moved rapidly 

 to and fro over the membrane ; when I held the pectoral motion- 

 less in a forward position the fish was unable to produce the 

 sound. Mobius (SB. d. Berlin. Akad. der Wissenschaften, 1889, 

 p. 999) attributes the sound in B. aculeatus of Mauritius to 

 stridulation between the postclavicle and a longitudinally grooved 

 area on the inner surface of each cleithrum ; both these bones 

 he states are in intimate relation with the air-bladder and a 

 portion of the lateral walls of the bladder is in contact with 

 the skin, which visibly shares in the vibratory movement of 

 the bladder when the sounds are emitted. I cannot disprove the 

 truth of this explanation, but it seemed to me as though the 

 pectoral actually set the membrane of the drum in vibration, and 

 in any case I satisfied myself that the movement of the pectoral 

 is necessary for the production of the sound. Batistes vetula, but 

 not B. buniva, also made another quite distinct sound of a grating 

 character apparently from the inside of its mouth, perhaps by 

 the rubbing together of pharyngeal teeth, but I was not able to 

 trace the origin of this sound more precisely. In both -species 

 the first dorsal spine is firmly locked when erected and can only 



