BTVALVIA 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 inhalent from the excurrent canal ; margins double, each bearing a 

 row of tentacular filaments, and at the base a row of black eyelets {ocelli) ; 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 striae, and in the very inequivalved species, of which P. maanmus 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 ; in some species, the animal when more advanced in age is capable of 

 considerable locomotion. 1 In Chemnitz ' Conch. Cab./ vol. vii, p. 261, vig. xi, there is 

 a woodcut which represents the imaginary animal of Pecten (opcrcularis ?) with two 

 distinct and separated siphons, prettily ornamented with setae or fibrillae 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, maximus 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. 



