226 ORGANIC REMAINS. 



seen described. It greatly resembles t. clavellata; but instead of 

 rows of knobs, it is adorned with ribs ; and these are not smooth, as 

 in t. costata, but each of them is surmounted by a row of very minute 

 tubercles. In the area, or front, the shell bears so strong a resem- 

 blance to t. clavellata, that it may possibly be no more than a variety 

 of that species, perhaps in a younger state ; these shells in the dogger 

 being smaller than those of t. clavellata in the oolite. Fig. 20 is the 

 beak of one of these shells. 



Cardium. Cockle. Many cockle-shaped shells occur in our 

 strata; but only a small number of species have been ascertained to 

 belong to this genus. 



C. edule, or the common cockle, is said to have been found in the 

 Malton quarries ; but it has not come under our notice. Fig. 4, PI. 

 VIII, is a beautiful cockle, from the grey limestone in that quarter, 

 resembling c. Icevigatuni; but it is of a rounder and more handsome 

 shape than any shells of that species which we have seen ; and is 

 rather more elegantly marked. The number of the ribs is fifty, or 

 upwards. Sowei'by's c. semigranulatum. Tab. 144, is akin to it. 



C. pectinatum, called also ceolicum, or one greatly resembling it, 

 is the most abundant species in our strata. It is striated longi- 

 tudinally in the front, but transversely on the disks ; as may be seen 

 in the specimen, Fig. 5, PI. VIII. It is found in great abundance in 

 the bands that run in the alum shale ; both the ironstone and sand- 

 stone bands. Peak, Boulby, and the fronts of the hills near Stokesley, 

 are the places where it is most common. Yet, though it is so plenti- 

 ful, we rarely meet with a specimen in which the shell is complete : 

 for the shell is usually gone, and only the heart-shaped jp,st left ; or 

 the latter comes out entire, while the former is broken to pieces, or left 

 in the matrix. The best specimens are obtained from the ironstone. 

 There is much variety in their size ; some being only half an inch 

 long, while others are an inch and a half, or upwards. The average 



