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 ujjwards 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 kno^vn. It has been quoted as an 

 inhabitant of the Palaeozoic Period, but the specimens found in the Coal Measures and 

 Mountain Limestone Rocks, present differences that arc considered as generically 

 distinct, and they have been separated by Prof. M'Coy under the name of Aviculopcden 

 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 haye 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 i-ender their correct appropriation a 

 task of no ordinary difficulty. 



1. Pecten maximus, Linnmts. Tab. IV, fig. 1, a—h, and Tab. VI| fig. 7, u — h. 



Lisler. Hist. Concli., lib. iii, par. 1, fig. 1 A, 1C87. 



OsTUEA MAXIMA. Linn. Syst. Nat., p. 1144, No. 185, 1767. 



— — Knorr. Dclices des Yeux, xiv*, fig. 1, 1766. 



— — Vonov. Brit. Shells, pi. 49, 1800. 



— — W. Wood. Ind. Test., p. 47, pi. 10, fig. 1, 1825. 



— — Broc. Conch. Foss. Subap., p. 572, No. 16, 1814. 

 Pecten maximus. Mont. Test. Brit., p. 143, 1803. 



— — Ckem. Conch. Cab., vii., p. 268, pi. 60, fig. 58.i, 1 782. 



— — Crouch. Int. Lam. Couch., p. 20, pi. 12, fig. 13, 1827. 



— — Brown. lUiist. Conch. Gr. Brit., pi. 32, fig. 1, 1827. 



— — Desk. 2il ed. Lam., t. vii, p. 129, 1836. 



— — Payr. Cat. Moll. Cors., p. 71, No. 132, 1826. 



— — N. Wood. Catalogue, 1840. 



