98 ON THE MINERALOGY OF THE 



ination. Indeed, in three species which I have com- 

 pared, I could not discover any defmiie specific 

 difference. 



9. Plagiostoma of Sowerby. — Some of the spe- 

 cies of this genus occur more frequently than any 

 other. They are readily distinguished by the hinge 

 line. 



10. Spirifer of Sowerby. — The S. cuspidatus, 

 tab. 120., occurs here. It is the Anomia cuspidata 

 of Martin, Linn. Trans, vol. iv. p. 45. tab. 3. 



There is one striking character which these pe- 

 trifactions exhibit, namely, their oval shape, by 

 which they may be distinguished from the recent 

 testacea. Neither this oval form, nor the oblique 

 position of the valves of the terebratulae, arise 

 from any bruise, but is quite natural, as I proved 

 by numerous observations. All these shells are 

 much incorporated with the rock, and the matter of 

 the shell is usually changed into calcareous-spar. 



I observed only one zoophyte, which did not oc- 

 cur in great abundance. It appeared to be conicor 

 tubular,— the surface reticulated, the pores round 

 in one variety, and lengthened in another. I could 

 not perceive in the centre any traces of an organic 

 structure. 



IIL^-Clay-Slate. 



The rocks of clay-slate occur to the south of the 

 two formations, whose characters have been given 



