44 MOLLUSCA EROM THE GREAT OOLITE. 



Arca, Linn., 1758. 



Arca rudis, Sow. Tab. V, fig. 12. 



Ctjcull^a rudis, Sow. Min. Con., t. 447, f. 4, 1824. 



Testa oblongd, subcylindricd, umbonibus magnis antemedianis approximatis, margine 

 cardinali elongate, marginibus aliis arcuatis, costis radiantibus numerosis anticis et 

 medianis acutis, posticis magnis irregularibus subdistantibus, nodosis, lineis concentricis 

 decussatis. 



Shell oblong, subcyliudrical, umbones large, contiguous, and placed anterior to the 

 middle of the valves, hinge border elongated, the other margins rounded ; radiating costae 

 acute, elevated upon the anterior and middle portions of the valves, much larger, irregular, t 

 more distant and nodose upon the posterior side, decussated by numerous concentric 

 hues. 



The posterior side of the shell is rather compressed, and has four or five very promi- 

 nent irregular knotted costae ; the concentric lines are very fine, and for the most part 

 indistinct. It occurs rarely in the shelly beds of the formation. 



Height, 6 lines ; length, 1 2 lines ; diameter through both the valves, 6 lines. 



Localities. Minchinhampton and Bisley Commons ; Ancliff, Wiltshire. 



Arca pulchra, Sow. Tab. VI, fig. 6. 



Arca pdlchra, Sow. Min. Con., t. 4"3, f. 3, 1824. 



Testa ovate-oblongd, convexd, umbonibus antemedianis subdistantibus, margine cardinali 

 elongate, margine infero parallelo, area angustd, lafcribus costalis radiantibus crebris sub- 

 ccqualibus, slriis concentricis tenuissimis indentatis. 



Shell ovately oblong, convex, umbones anterior, separated, hinge margin elongated, 

 with the inferior margin of the valves parallel to it, area narrow ; the sides of the valves 

 are ornamented with densely arranged radiating little ribs, these are rather unequal in 

 size, but radiate in every direction, and are indented by fine closely arranged concentric 

 striatums. 



The example of this species figured in the ' Mineral Conchology,' represents the 

 immature stage of growth in which the concentric striations are scarcely distinguishable, 

 and the general form has not acquired the full degree of convexity. In the description 

 of Arca pulchra, Mr. Sowerby states that it is " nearly twice as wide as long ; the striae are 

 very uniform and close together ; the valves are rather flat in the middle ;" and further 

 remarks — " Although there is hardly any appearance of a sinus in the margin, this is 

 placed as an Arca because it has no transverse elongated teeth in the hinge, those nearest 

 the extremities being longitudinal; it is, however, one of the links that unite the two 

 genera." 



