BIVALVE SHELLS. 219' 



anterior slope, at the opposite end of the channel between the folds 

 of the umbones, are two similar knobs, rather larger, forming a con- 

 siderable swell. In the channel between these two pairs of knobs 

 runs the hinge i which, were there an opportunity to examine it, 

 might perhaps be found to be that of an area rather than a mytilus ; 

 the shell having some relation to the aixa modiolus. 



Mya. Gaper. Various shells that may be assigned to this 

 genus, are found in our strata. Among these we may place No. 6 

 (PI. VII), which is from the Malton oolite ; or rather, from the grey 

 limestone under the oolite. It seems akin to 31. ovata. Other oval 

 shells occur at Malton, resembling the M. jnctorum. The unto 

 acutus of Sowerby, Mineral Conchology, Tab. 33, may be noticed 

 under this genvis ; though its shape rather entitles it to be reckoned 

 a donax. It is very common in some of our beds of ferruginous 

 sandstone, particularly in the Kettleness and Staiths bands, as found 

 in the front of the Cleveland hills. A cast corresponding with the mya 

 viandibula of Sowerby, Min. Conch. Tab. 43, has been observed in 

 the oolite. In PL VIII. Fig. 7, we have given the unio Listeri of the 

 same author. Tab. 154. It is from the aluminous strata, where it is 

 pretty common ; but it may rather be regarded as a tellina or a 

 donax, than a mya or imio. 



Donax. Wedge-shell. If the shell last mentioned may not be 

 assigned to this genus, it may at least claim that which we have given 

 in PI. VII. Fig. 8. This shell is from the Staiths band, in which, and 

 in some other parts of the aluminous strata, it is very common. In 

 shape it resembles the D. denticulata, but has the margin entire, and 

 has a depression at the truncated end, immediately beyond the beaks. 

 It is slightly striated longitudinally. In many specimens, one of the 

 valves projects a little beyond the other, at the beaks; and in many, 

 there is a contraction or indentation of the shell in the posterior edge : 

 but as these marks vary in different specimens, being sometimes 



