336 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
P. vitreus (Chemnitz), P. granlandicus, Sowerby, and four 
varieties of Pecten opercularis—var. lineata, da Costa, var. 
tumida, Jeffreys, var. elongata, Jeffreys, and var. audouinz, 
Payraudeau. 
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. maximus and P. opercularis, but especially 
the latter, are gregarious; and in various places round the 
British Coast beds of scallops exist where P. opercularis 
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. opercularis, 
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 ntost interesting peculiarities of the genus, 
and, moreover, certain features in the anatomy of the 
molluse have, in all probability, been modified owing to 
this habit. Pecten opercularts swims much more 
frequently and for a longer period than P. maaimus, and 
if specimens are kept in aquarium tanks, it 1s 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 mignt be expected, 
the animal moves with the ventral edges of the shell 
foremost. The mollusc, which has been lying on one of 
