246 ORGANIC REMAINS. 



Shells of this genus are found in all parts of our aluminous stra- 

 ta, but especially in the main bed of alum shale. The best specimens 

 are procured from the pyritous or lias nodules ; but great numbers 

 are found disengaged. In several places near Whitby, the scar seems 

 paved with them ; and the pavement has a rich appearance where 

 they are coated with shining pyrites. Most of the shells that are 

 found without a nucleus are imperfect, the inner whirls being decay- 

 ed, or the whole shell being broken and flattened. It is observable^ 

 that they are almost always found in the plane of the strata, their flat 

 surfaces being parallel to the horizontal partings ; a remark which 

 applies to our fossil oysters, and other flat shells, as well as to the 

 flat pieces of fossil wood. 



Ammonites are found also in the lower chalk, the upper shale, 

 the second shale, the oolite, the grey limestone, the calcareous sand- 

 stone, and in a few of the ironstone beds. We have observed about 

 thirty different species, of most of which we have given specimens in. 

 Plates XII, XIII, and XIV. 



No. 3, PI. XII, is the most plentiful of our ammonites, and is 

 hence called by Sowerby ammonites communis. Tab. 107, Fig. 2, 3. 

 The aperture is round ; and its edge, or lip, when entire, is reflected 

 or partially folded back, like the edge of a trumpet or a bell ; as is 

 the case in some species of the helix family. The ribs are numerous, 

 and generally sharp, and eacli of them at the back is divided into two, or 

 a short rib comes between every two long ribs. The shell has generally 

 five or six turns \ but in this respect, as well as in the closeness and 

 sharpness and curvature of the ribs, there is much variety. This 

 species abounds in the main bed of alum shale. It seldom reaches 

 four inches in diameter. — A shell, apparently of this species, or ap- 

 proaching very near to it, occurs in the lower chalk, and in the upper 

 shale, where it is generally in a highly nacrous state. — An ammonite 

 greatly resembling this species, but much flatter, and often of a larger 



