BIVALVE SHELLS. 239 



Fig. 13, PI. IX, represents a small shell from the oolite, of an 

 oblong form, and singularly marked; being striated longitudinally 

 towards the base, but transversely near the beaks. It is diificult to 

 say, whether it is an ostrea, or a chama, or neither. This remark 

 applies also to a very large shell, found in the Flaraborough chalk ; 

 very similar in shape, and marked with coarse transverse furrows, 

 that are deepest near the beaks. This is the largest of our oysters, if 

 we may include it in the family. One of our specimens has measured, 

 when entire, about a foot in length, and seven inches in breadth. 



No. 3, PI. X, is a shell from the oolite, which also we hesitate to 

 place in this family. It has the ruggedness of an oyster, with the 

 libs of a pecten ; which last it agrees with in shape, but we have not 

 discovered any ears. The margin, like the surface, is very irregularly 

 waved. Some specimens exceed four inches in diameter. 



There are many ostracites, in the bands of the aluminous strata, 

 having one valve remarkably gibbous. They seem to be of two 

 different species ; one being very small, roundish, and wrinkled ; 

 the other oblong, nearly smooth, and two or three inches long. The 

 latter resembles the shells of the next genus. 



GryphvEA. JBoat-oyster, or ynillers thumb. Gryphites are found 

 in our strata in considerable quantity; particularly in the lowest shale, 

 as in Robin Hood's Bay, at Boulby, and at Redcar rocks ; and in the 

 calcareous sandstone, as at Scarborough, Filej', and Rowlston scar. 

 Like the ostracites, they are seldom sparry, but retain the appearance 

 of recent shells. Fig. 23, PI. Vlll, represents the most common 

 species ; corresponding with that of Parkinson, III. PI. XV. Fig. 3 ; 

 which Sowerby, Tab. 112, names g. incurva. It occurs in various 

 states ; small, and large ; smooth or slightly striated, and strongly 

 wrinkled. Numbers are often found clustering together. 



Fig. 4, PI. X, is Sowerby's g. dilatata, Tab. 149. It occurs in 

 the hard sandstone at the foot of Scarborough castle, in Rowlston. 



