341 



On the Occurrence of Plicatula laevigata of cVOrhigny 



in the Middle Lias of Gloucestershire, by 



Frederick Smithe, LL.D., F.G.S., etc. 



The Plicatula of Lamarck (Harpax of Parkinson) is a genus 

 which embraces both fossil and recent species, and is included 

 in the family Ostreidse of the acaleph mollusca. 



As a genus, it was established by Lamarck in 1810, who based 

 it upon his observation of the hinge character of Spondylus 

 plicatulus, a mollusc still living in the Red Sea. The animal 

 of Plicatula is an inhabitant of warm seas, resembling Spondylus 

 in every essential respect, and is only like Ostrea in that the 

 foot is obsolete or nearlj^ so ; for the shell being always attached, 

 the animal is stationary and needs no power of locomotion. 

 Still, between Lamarck's type and the recent shell, there exists 

 merely a general resemblance of character. The teeth are two in 

 number, in the form of an inverted Y, and are borne by the 

 right valve. Confining our observations to the liassic species 

 of the genus Plicatulce, we first notice that our most familiar 

 form, is that of the Plicatula Spinosa, a small protean shell 

 which ranges through nearly the whole of the Lias formation, 

 adapting itself with versatile character to whatever surface may 

 be available. On the continent of Europe, indeed, the little P. 

 Spinosa is so abundant at particular places, Cerny, for instance, 

 in the Middle Lias of Upper Normandy, that the beds in which 

 it predominates are designated by M. Jules Marcou, the 

 " Marnes a plicattdes." Leaving this comparatively pigmy shell, 

 we now come to the fine species which I was fortunate enough, 



