TO GEOLOGY. 89 



Forty-nine species of this genus have been observed in 

 the strata of Great Britain, from the Carboniferous Lime- 

 stone to the Crag three only in the London Clay. Many 

 are described from the Continent, by Nilsson, Heeninghaus, 

 &c. as existing in the Cretaceous Group. M.M. Brogniart 

 and Defrance have described some from the environs of 

 Paris, where they have been found in the Supercretaceous 

 rocks. M. Deshayes gives us sixty for the Tertiary. 

 Twenty-six are from the Pliocene of Sicily and the Sub- 

 appennines, and ten from the Paris basin. In the Green 

 Sand of New Jersey and Delaware, Dr Morton has ob- 

 served two or three species. Mr Say, from the Tertiary 

 of Maryland, has described four. 



GENUS PLICATULA. Lamarck. 

 P. Mantilli.* Plate 3. Fig. 68. 



Description. Shell oblongo-trigonal, irregular, compress- 

 ed, ribbed, transversely imbricate, furnished with external 

 and internal ribs, the latter most numerous ; substance of 

 the shell thick ; teeth striate ; cicatrix round. 



Diam. .3, Length .7, Breadth .8, of an inch. 



Observations. This species, as sometimes among the 

 recent ones, is so irregular in outline as to present almost 

 all the forms between the circle and triangle. It is dis- 

 posed to be angular above and round below. The small 



* Named after the author of the " Geology of Sussex." 

 M 



