BIVALYIA. 47 



unequal, and are ornamented with about half a dozen (5-7) primary rays, generally imbri- 

 cated j the two upper often coalesce, particularly in the right valve, beneath which there 

 is a deep sinus. 



P. opercularis is mentioned in Mr. Prestwich's list of species, in his paper on the 

 London clay (p. 405, Geol. Journ.), as from Bracklesham. This may perhaps have been 

 a dead specimen of the recent shell, or perhaps a specimen from the newer Tertiaries of 

 that locality; or it may have been a variety of '30-radialus. I have seen nothing from the 

 older Tertiaries that can be united with the recent species, opercularis. 



Pecten subreconditus, D'Orb. (Prod. Pal., 1850, p. 393, No. 1106), is, I think, not an 

 Eocene shell. He refers to ' Sow. Min. Conch.,' pi. 575, figs. 5, 6, which are representations 

 of a Crag specimen of opercularis. P. subreconditus, Pictet (pi. 83, fig. 10), who refers to 

 IVOrbigny, is probably the same. 



LIMA, Bruguiere, 1792. 



Generic Character. Shell ovate, equivalve, generally inequilateral ; the pedal region 

 oblique and gaping. Sometimes the shell is straight and equilateral ; exterior occasionally 

 smooth, more often radiately ribbed or striated, rough or squamous. Hinge area triangular, 

 extended into auricles ; connexus bipartite, with central pit ; adductor impression large, 

 lateral, and double ; two small pedal scars. 



Animal has the mantle margins disunited, and fringed with a double row of tentacular 

 filaments, the inner one long and pendulous ; ocelli inconspicuous ; foot small, finger-like, 

 furnished with a byssal groove. 



Several divisions have been made of the shells that will come into the above 

 formula. 



Limea has been proposed for those species which have a row of teeth or crenulations 

 upon the hinge margin, and Limatula for those which are equilateral and closed. 



In general, the oblique species {Lima proper) have one side straight, and the other 

 rounded, with one opening ; but this is by no means constant, as there is often a large gape 

 on both sides. The shell of Limatula has sometimes a row of crenulations, while the same 

 form of shell is known to be without them. Limea} has teeth on each side of the cartilage- 

 pit upon the hinge-margin ; these are sometimes in a rectangular position, at others they are 

 placed obliquely to the hinge-line, and these are generally upon shells that are equilateral 

 or nearly so, but they are not confined to that form of shell ; and in the oblique species there 



1 Goldfuss has figured two species of Secondary fossils, one from the Lias, L. acuticosta, pi. 107, 

 fig. 8, with large dentations. 



