BIVALVE SBELLS. 217 



numbers of valves are found single, and many of them water-worn ; 

 so that these must have been imbedded a considerable time after the 

 <ieath of the shell-fish, and after many of them had been worn among 

 sand and gravel, like the dead shells which we pick up on the shore. 

 Entire seams of woi'n shells are sometimes met with, though in 

 general we find both complete and single shells in the same stratum. 

 Some of the strata too abound with minute fragments of shells, with 

 which whole shells are often intermixed. 



It is also worthy of remark, that several shells, or parts of shells, 

 are found preserved, not petrified ; and this is particularly the case 

 in some of the clay strata, especially in soft marly clay, such as we 

 find in the calcareous beds under the oolite. 



These things being premised, we shall now give a list of the 

 principal genera and species of bivalves, belonging to our strata: 

 which we shall do, without arranging them exactly in the order of the 

 Linnsean system. 



SoLEN. Razor-shell. Several species of solen occur in the oolite, 

 the calcareous sandstone, and the lowest shale. Among these we 

 may distinguish the S. siliqica, the S. ensis, and a smaller straight 

 shell resembling the S. legumen. A specimen of the S. ensis, or 

 scymetar solen, is represented PI. VII. Fig. 1. It is from one of the 

 hard seams in the lowest shale, at Robin Hood's Bay; and, though 

 not quite entire, shews the striae, and even something of the colour, of 

 the recent shell. The oolite abounds most with shells of this genus, 

 especially in the quarries of Pickering and Thornton, where lai'ge 

 specimens of the solen siliqua are often met with. 



Mytilus. Muscle. Of this genus, one of the most singular 

 species is the mytilus lithophagus, or stone^eater; which, like the 

 pholas, burrow s in rocks, and not imfrequently in corals. It occurs 

 in the oolite, or rather in masses of coral imbedded in the oolite, at 

 Ayton, Malton, and other places. Fig. 5 (PI. VII) is an interesting 



3 H 



