22 MOLLUSCA FROM THE CRAG. 
ages an opening in one valve through which a byssus of considerable magnitude 
might have passed ; others fix themselves to rocks or foreign bodies by the spinous or 
imbricated processes of their valves; when fixed like P. pusio, it is by the right valve, 
which is also the one wherein is left an opening for the byssus, and the one also like 
P. maxima, which the animal buries in the sand, it may, therefore, always be considered 
the lower valve : this is sometimes the most convex, while in those that are free, the 
greater convexity is generally in the left or upper valve, which, in the living shell, is 
the more highly coloured. 
This is purely a marine genus, and in the recent state has a very extended 
geographical distribution, being found in almost all parts of the world, while its 
vertical range is also considerable, inhabiting the seas at various depths ; it is also of 
great antiquity, species having been found as low in the Secondary Series as the Lias, 
and are continued upwards through nearly every period to the present time; it is 
largely developed in the newer Tertiaries, and is exceedingly abundant as an existing 
genus, upwards of a hundred species being already known. It has been quoted as an 
inhabitant of the Paleozoic Period, but the specimens found in the Coal Measures and 
Mountain Limestone Rocks, present differences that are considered as generically 
distinct, and they have been separated by Prof. M‘Coy under the name of dviculopecten 
differing from the true Pectens in the absence of a cartilage-pit, and in the inequalities 
of the auricles being reversed, thereby connecting it with Avicula. 
It is to be feared, that many of our Tertiary specimens have been erected into 
species without sufficient claim to such distinction, and that several will be found upon 
further examination to be merely variations of form and sculpture of those which are 
perhaps more than commonly disposed to deviate from what may be considered as the 
typical form of long and well-known recent species. 
This genus flourished most abundantly in the Crag Seas, and the modifications in 
the ornamental portions of most of the species render their correct appropriation a 
task of no ordinary difficulty. 
1. PecTEN MAxiMus, Linnaeus. Tab. IV, fig. 1, a—6, and Tab. VI, fig. 7, a—hb. 
Lister. Hist. Conch., lib. iti, par. 1, fig. 1 A, 1687. 
Ostrua Maxima. Linn. Syst. Nat., p. 1144, No. 185, 1767. 
—_ — Knorr. Delices des Yeux, xiv*, fig. 1, 1766. 
= — Donov. Brit. Shells, pl. 49, 1800. 
— —_ W. Wood. Ind. Test., p. 47, pl. 10, fig. 1, 1825. 
— — Broc. Conch. Foss. Subap., p. 572, No. 16, 1814. 
Prcren Maximus. Mont. Test. Brit., p. 143, 1803. 
— os Chem. Conch. Cab., vii., p. 268, pl. 60, fig. 585, 1782. 
—_ — Crouch. Int. Lam. Conch., p. 20, pl. 12, fig. 13, 1827. 
— — Brown. Mlust. Conch. Gr. Brit., pl. 32, fig. 1, 1827. 
—_ a Desh. 2d ed. Lam., t. vii, p. 129, 1836. 
= _ Payr. Cat. Moll. Cors., p. 71, No. 132, 1826. 
— — S. Wood. Catalogue, 1840. 
