Sowerby's Mineral Conchologi/. 283 



readers as are interested in the details, to the work itself, on the 

 fidelity of which they are aware that the fullest reliance may be 

 placed. 



The first of these is distinguished by the name of Paciiymya. 

 It is thus ciiaracterizcd ; " Shell bivalved, transversely elongated, 

 very thick, sub-bilobatc, with the l)eaks near the anterior ex- 

 tremity. Ligament partly immersed, attached to prominent pro- 

 cesses, or fulcra." It bears a considerable analogy to Modiola in 

 the position of the beaks, the elongated form of the valves, and 

 the partial separation of the anterior portion into an imperfect 

 lobe. From the thin shells which are allied to Mj/liliis, it is dis- 

 tinguished by its comparatively short ligament fixed upon a strong 

 prominent part of the shell within the edge, utterly unlike their 

 linear ligaments afiixed to the narrow margins themselves. It is 

 founded on a species from the lowest part of the Chalk formation 

 near Lyme Regis, to which the trivial name of P. G/);as has beea 

 given. It appears probable that other fossil shells, hitherto de- 

 scribed as Modiolce, will, when better known, be found to belong 

 to this genus. 



The other new genus is also founded on a bivalve shell. It is 

 denominated Thetis, " An equivalved subequilateral bivalve; 

 more or less orbicular, and convex ; ligament marginal ; three or 

 four small acuminated teeth about the hinge; the line of attach- 

 ment of the mantle ? has a deep sinus extending nearly to the 

 beak; muscular impressions rounded, small, distant from the 

 hinge. Ligament external." It differs from Mactra by the 

 absence of remote teeth, and by the want of an internal ligament; 

 and from TeWna by its margin not being curved laterally. The 

 fossil shells composing it have frequently been referred to Venus^ 

 but are totally distinct. They exhibit no lunette, nor other ex- 

 ternal mark. Two of them are figured under the names of T. 

 major, and T. viinor ; the former is from the Sandstone of the 

 Greenstone formation, in various localities, and the latter from 

 the Greensand itself. 



