400 THE MEDALS OF CREATION, Cap. Xl. 
Several smooth species of this genus are found in the © 
Chalk,* Oolite, and Lias. A very large species (P. gigantewm), — 
sometimes ten inches in diameter, abounds in the Lias _ 
(Ly. p. 274). It is somewhat depressed in form, with the 
surface slightly striated ; each valve has a pointed beak, — 
with two lateral expansions, or ears, as they are termed by — 
conchologists. ’ 
PuicaTULA, is another genus of this family, of which — 
there are three British fossil species. A delicate shell, with — 
slender depressed spines (P. inflata. Foss. South D. pl. — 
XXvi.), occurs in the Chalk Marl. The recent species are ~ 
natives of the seas of warm climates. ; 
A 
PEectEeN.—The common scallop-shell will serve as a type 
of this genus. The animals of these shells, unlike the 
oysters, have the power of locomotion, and when in the 
water, may be seen moving with rapidity, and flapping their © 
shells to and fro with great activity. Numerous species are 
found fossil. In the Pliocene, and other marine tertiary 
deposits, Pectens abound; in the White Chalk there are 
several elegant forms (see Yoss. South D. plate xxv.); many 
kinds in the Oolite and Lias ; and several in the Devonian ~ 
strata. . 
A large Mediterranean species (Pecten Jacobeus, Ly 
p. 152 ) occurs in the Pliocene strata of Palermo, in every — 
stage of growth, and as perfect as if recent. The Chalk 
and Shanklin sand contain a small inequivalved Pecten, the 
lower valve of which is convex, and pentangular, the upper 
flat, and both strongly ribbed, or pectinated; it is named 
Pecten quinquecostatus (Foss. South D. pl. xxvi. Ly. p. 212); 
and in the cretaceous strata of North America a variety of 
this species is found. 
In the Chalk Marl a large and beautiful Pecten (P. Bea- | 
* See Foss. South Downs, plate xxvi. 
