BIVALVE SHELLS. 215 



and strictly speaking have no valves at all. The last class is sub- 

 divided into UNILOCULAR, having only one chamber or cavity; and 

 MULTILOCULAR, having several chambers. 



The raultivalve class is very small, containing only the lepas, or 

 harnacle; and the chitoti; to which some add the pholas, which, 

 besides two principal valves, has small accessory valves upon the 

 hinge and posterior slope. A good specimen of the lepas tintinnabu- 

 lum, a shell that exists in the recent state on the British shores, is 

 figured in the Scarborough Catalogue, PI. I. Fig. 1. It was found 

 adhei'ing to a fragment of a pectenite, in the calcareous sandstone at 

 Scarborough castle. Having seen few other interesting shells of this 

 genus, and none of the chiton or the pholas* in our strata, we pass 

 on to the next class, viz. 



BIVALVE SHELLS. 



Fossil shells of this class occur in immense numbers, many of 

 them corresponding with those which our seas still produce, but a 

 much greater number that have no recent analogue on our shores ; 

 not a few of which have never been discovered, in the living state, in 

 any part of the world. Before attempting any enumeration of genera 

 and species, we would make a few preliminary remarks. 



Great difficulty is experienced in arranging our fossil bivalves. 

 There are many that can seldom be obtained in a complete state; 

 and in such as are complete, it is a rare thing to find an opportunity 

 of examining the hinge, from which the discriminating characters of 

 bivalves are principally drawn. Nor is it easy to ascertain, whether 

 a shell has been close, or gaping; and whether its sinuosities, 



* The pholas, in its recent state, abounds on our shores. For an account of that singular 

 animal, which burrows in the alum rock and the lias beds, see History of Wiiilby, II. p. 799. 



