BIVALVIA 37 
elongately triangular; impression of the adductor somewhat obscure, excentric; double 
pedal impression visible in right valve, obsolete in the left; impression of the mantle 
not well defined. 
Animal, with the edges of the mantle disconnected all round except at one part, which 
serves to separate the imhalent from the excurrent canal; margins double, each bearing a 
row of tentacular filaments, and at the base a row of black eyelets (oced/7) ; foot small, 
sub-cylindrical or digitiform, grooved, and byssiferous in the young state. 
This constitutes a well-marked group, although the species present considerable 
diversity of character in their exterior ornaments. The valves are generally covered with 
radiating ribs or stria, and in the very inequivalved species, of which P. maximus is the 
type, the rays are large and elevated, while at the other extremity, such as P. pleuronectes, 
the valves are nearly smooth, though still preserving the outward form and other 
characteristics of the genus. 
The foot of the animal is very small and incapable of being extruded, and probably 
the only use it makes of this organ is to spin a byssus, which is employed principally in the 
young state; im some species, the animal when more advanced in age is capable of 
considerable locomotion.’ In Chemnitz ‘Conch. Cab.,’ vol. vii, p. 261, vig. xi, there is 
a woodcut which represents the ¢maginary animal of Pecten (opercularis ?) with two 
distinct and separated siphons, prettily ornamented with sete or fibrillee projecting beyond 
the margin of the shell, and a large geniculated foot is protruding to some distance in an 
opposite direction. 
There is always more or less difference in the magnitude of the two valves in the shells 
of this genus, but it is in those species which lie habitually on one side that the great 
inequality is most distinct; the convex or tumid valve, being buried in the sand or mud, 
is usually colourless or fainter in ornament than the upper or flat valve, which is always 
exposed to the light. 
The genus has a very extensive vertical range, the British species having been obtained 
alive by Mr. M‘Andrew from the shore to the depth of 150 fathoms, though the most 
frequented habitats of two edible species, mawimus and opercularis, are upon banks that 
do not extend beyond the range of thirty fathoms. Its geographical range is very 
extensive. 
1 The stationary habit of this genus is generally by means of a byssus; there is one species 
P. pusio of British authors, which fixes itself by the outside of the shell, like the oyster, but, unlike that 
genus, it employs the right valve, or that in which is left a sinus for the byssus; sometimes the whole of 
this large valve is soldered to the rock. In this it is connected with Hinnites (Defr.), which, how- 
ever, is very different in all other respects. I have not seen a fossil Pecten possessing such a habit. 
