406 THE MEDALS OF CREATION. Cuap. XI. 
the common Arvca.* (See Min. Conch. tab. 27). At Plum- 
stead, near Woolwich, a smaller species is found; and also 
occasionally with the oysters at Bromley. 
Nucurta.—Several species of a small elegant bivalve, 
related to the preceding, but distinguished by having two 
rows of teeth on the hinge, diverging from an interspace 
between the beaks, are found in the Crag and other tertiary 
deposits (Jin. Conch. tab. 180, 192). Two species occur 
in the Galt (Foss. South D. pl. xix. fig. 5, 6, 9), at Ring- 
mer, Folkstone, Bletchingley, &c., sometimes with the shell 
perfeet, but generally in the state of casts composed of 
indurated clay, and having impressions of the muscles and 
of the two rows of hinges teeth, The shell of one species is 
marked with fine transverse grooves, or striz (V, pectinata) ; 
the other is of a flattened ovate form, and the surface 
smooth (ZV. ovata). 
The most beautiful species of Vucula are the WV. biwirgata 
of the Galt of Folkestone, and WV. Cobboldice of the 
Norwich Crag. 
The species of Vucula with the posterior side produced 
into a long beak have been separated under the name 
Leda; they have a pallial sinus, indicating a siphon to the 
mantle ;— 
e.g. Nucula ovum . . . Alum Shale. 
— claviforms .  Lias. 
» — attenuata . . Coal Shale. 
— arctica. . . Norwich Crag. 
4 
Pinna.—The common large Pinna, of the Mediterranean, 
is well known, and differs so entirely from other shells, as to 
* The species so abundant at Bognor, is P. brevirostris, Min. Conch. 
tab. 472, Ihave seen a block of the limestone, in which, spread 
over an area of a foot square, there were upwards of fifty specimens 
lying in relief. 
