p. vitreus (Chemnitz), P. groenlandicus , Sowerby, and four 

 varieties of Pecten opercularis—vaT. lineata, da Costa, var. 

 tumicla, Jeffreys, var. elongata, Jeffreys, and var. audouini, 

 Payraudeaii. 



For systematic descriptions of these species and 

 varieties reference should be made to Forbes and Hanley's 

 " British Mollusca," and Jeffreys' " British Conchology." 



Both P. viaximus and P . oj^crmdaris, but especially 

 the latter, are gregarious ; and in various places round the 

 British Coast beds of scallops exist where P. ojjercularis 

 can be obtained in thousands by dredging. Both species 

 prefer a sand or gravel bottom, but sometimes they occur 

 on mud. The depth of the great bed of P. 02)evndaris, 

 situated off Port Erin at the South-west end of the Isle 

 of Man, is about 17-22 fathoms, and all the specimens of 

 both species used in preparing this Memoir came from an 

 average depth of about 20 fathoms. 



BIONOMICS. 



The animal is found lying free, neither adherent by 

 the shell nor by a byssus. Locomotion, however, is 

 carried on, not by the usual Lamellibranch methods of 

 creeping or leaping, but by spasmodic swimming. This 

 is one of the most interesting peculiarities of the genus, 

 and, moreover, certain features in the anatomy of the 

 mollusc have, in all probability, been modified owing to 

 this habit. Pecten ojjercidans swims much more 

 frequently and for a longer period than P. ma.vimus, and 

 if specimens are kept in aquarium tanks, it is quite easy 

 to follow their movements and make out the structures 

 involved in this curious method of progression. It strikes 

 one at once that, contrary to what might be expected, 

 the animal moves with the ventral edges of the shell 

 foremost. The mollusc, which has been Iving on one of 



