BIVALVIA. 45 



Localities. The shelly beds of the Great Oolite throughout the Minchinhampton 

 district ; Ancliff, Wilts ; Ponton, Lincolnshire. 



Arca Kilverti. Tab. V, fig. 10. 



Testa oblongd aut subrhomboidali , umbonibus mediants contiguis ; lateribus compressis ; 

 bast arcuatd ; superficie bipartitd sulco lato ; costulis radiantibus {circa 50) stibnodosis, 

 lineis concentricis crebris decussatis. 



Shell oblong or subrhomboidal, umbones moderately large, mesial, and contiguous; 

 the sides of the shell compressed, the lower margin curved ; the surface divided into two 

 parts by a wide superficial sulcation; radiating lines or costas (about 50) slightly nodose, 

 and decussated by fine, closely arranged concentric lines. 



Upon the sides of the shell the costae are more widely separated, and the interstitial 

 spaces sometimes disclose a smaller rib. It ranks as one of the more rare Great Oolite forms, 

 the figure nearly agrees with Arca bipartita, Roemer, 'Nordd. Ool.,' tab. 14, fig. 12 ; but 

 that species is destitute of the concentric lines. 



Height, 3 lines ; length, 6 lines. 



Locality. Minchinhampton Common. 



Named in compliment to Mr. Kilvert, of Bath. 



Arca tenuitexta. Tab. V, fig. 9. 



Testa parvd, ovato-oblongd, umbonibus obliquis antemedianis subdistantibus ; marginibus 

 rotmidatis; dorso convexo, superficie mediano sulco lato impressd; lineis radiantibus crebris 

 minutis aliis concentricis decussatis. 



Shell small, ovately oblong, umbones oblique, anterior to the middle of the valves, and 

 separate ; hinge margin of moderate length, its extremities rounded, base slightly sinuated 

 by a wide but superficial sulcation which descends from the umbo ; the entire surface is 

 covered with radiating, closely arranged, depressed, very fine fines, crossed by others 

 concentric and closely arranged. 



The genera] figure has a considerable degree of convexity, and the hinge margin is 

 very short; the radiating and concentric interstitial spaces are so minute that they 

 resemble fine punctuations. It ranks as one of the smallest and most rare of the Great 

 Oolite Arcacea ; it is found in the planking and contemporaneous white stone of 

 Bussage. 



Localities. Minchinhampton and Bisley Commons. 



Arca Prattii. Tab. V, fig. 3. 



Testa subrhomboided, umbonibus antemedianis contiguis, area angustatd ; latere antico 

 margine rotundo, latere postico producto, angulo obliquo declivi ; costulis radiantibus 



