416 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



Onos spp., Eisso. — The Rocklings. 



Two species of rockling are known to occur in the 

 Irish Sea, Onos mustela, five-bearded rockling, and 

 Onos tricirratus , three-bearded rockling. The former is 

 the species most frequently met with, and is probably 

 the one generally represented by eggs and young stages 

 in the plankton of the area. The third species, Onos 

 cimbrius, four-bearded rockling, may also occur, 

 although it has not been recorded by any observer from 

 the central area of the Irish Sea. The summary of the 

 results of the six years' investigation of the plankton 

 from the south-west of the Isle of Man shows that the 

 eggs belonging to one or other of the two species first 

 mentioned may be present in the plankton throughout 

 almost the whole year. The spawning period of the 

 five-bearded rockling, according to Mcintosh and 

 Masterman,* lasts from April to August, and the three- 

 bearded rockling from November to January. The size 

 of the egg of the five-bearded rockling, according to these 

 authors, is 0'72 mm., and the oil globule 0'0825 mm. 

 The egg of the three-bearded rockling measures 0"74 mm. 

 and its oil globule 0'218 mm. The only difference, 

 therefore, between the eggs of the two rocklings is in the 

 size of the oil globules. In 1907 rockling eggs were 

 present in the plankton from February 5th to 

 September 19th, with the exception of July, when none 

 were observed. The eggs occurred from January 4th in 

 1908 to August 7th without any interruption. In 1909 

 they were present almost continually from January 2nd 

 to September 21st. The distribution during 1910 was 

 much the same as in 1909, except that they persisted for 

 nearly three weeks longer. The eggs were first noted on 



* British Marine Food Fishes. London, 1897. 



