BIVALVE SHELLS. 231 



with rows of oblong knobs, which appear to have terminated in 

 minute spines, now broken off. 



In the oolite at Old Malton, we found a fragment of a large shell 

 of this genus, with very deep ribs covered wdth scales; but whether it 

 belongs to the c. gigas, or to some other species, we cannot decide. 



As the handsome pyritous shell, Fig. 15, PI. VIII, approaches the 

 shape of c. calcarata, we may notice it here, though it very probably 

 may belong to another genus. It is imbedded in a pyritous nodule 

 from the alum shale, and is only imperfectly seen. It is finely striated 

 or ribbed, with transverse concentric lines. 



Arca. ArJc. Of 'this kind of shell, only a few species are 

 known to exist in our strata. The most common is that figured in 

 PI. VIII. No. 21, which abounds in some parts of the alum shale, 

 especially near Whitby east pier. Mr. Parkinson placed it under the 

 genus tellina, Avith some hesitation. Org. Rem. III. p. 192. PI. XIII, 

 Fig. 4 ; but the hinge which we have figured. No. 21 a, shews it to 

 be an arca; and to belong to that division in which the teeth are 

 in a broken line. It corresponds so exactly with the recent British 

 shell, the arca rostrata, both in its shape, its size, and its striee, that 

 there can be little doubt of their identity. There are two muscular 

 impressions in each valve, one at each end of the hinge ; in which 

 respect, as well as in its external form, it appears to coincide with 

 the recent shell. The fossil shell being in a state of spar, is thicker, 

 as may be expected, than the recent shell usually is; but it has 

 evidently been a Aveak shell, great numbers of the specimens being- 

 much bruised or compressed. It often retains something of the 

 original horny colour, but in some of the hard nodules it has a 

 bluish tinge. 



No. 22 is the right valve of a large ark, from the oolite, which 

 does not agree with any species that we have seen described. Its 

 shape is somewhat ovate and gibbous; and, like the area noce, it has 



