BIVALVIA. 15 



Plicatula tuberculosa. Tab. II, fig. 4. 



Testa ovato-orbiculari siibobliqud, convexo-pland tuberculis obtusis numerosis in lineis 

 radiantibus fere dispositis; umbonibas losvigatis sine tuberculis. 



Shell very irregular, ovately orbicular, rather oblique, and depressed with numerous 

 obtuse tubercles, for the most part disposed in radiating imperfect lines, umbonal extremity 

 smooth, destitute of tubercles. 



The round blunt warty tubercles have an aspect very different from the spines, either 

 fistulous or pointed, with which the Plicatulce, are for the most part furnished. The 

 tubercles usually increase in size towards the ventral border of the valve, but in a very 

 irregular manner, which, together with the very unequal and irregular surface of the valve, 

 produces a very confused appearance ; thus it happens that no two specimens can be found 

 which nearly resemble each other ; occasionally, the tubercles may be discovered approxi- 

 mating to the fistulous character ; the margins of the valves are plicated in a very irregular 

 manner. It is somewhat rare, but occurs in several of the shelly beds. 



The two diameters across the valves are nearly equal, and rarely exceed 10 lines. 



Localities. Minchinhampton and Bisley Commons. 



Plicatula fistulosa. Tab. II, fig. 5. 



Testa ovato-orbiculari depressd, costulis fistulosis radiantibus irregularibus interstitialibus 

 profundis. 



Shell ovately orbicular depressed, costae radiating irregular, with numerous irregular 

 prominent fistulous spines ; interstitial spaces deep. 



About 12 or 13 costae are distributed around the circuinference of the valve ; owing to 

 the delicacy of the test, it is very rarely that a perfect specimen can be obtained. 



Locality. Minchinhampton Common. 



Avicula, Lam. 1801. 



Gen. Char. Shell inequivalve, eared, base transverse, straight, its extremities produced 

 and forming auricles, left valve convex, umbone prominent ; right valve smaller, flattened, 

 its umbone depressed, and nearly obsolete. Hinge linear with a small indistinct tooth 

 in each valve beneath the umbones, and a lengthened marginal ligamentiferous area. One 

 rounded subcentral muscular impression in each valve, with a series of smaller ones in a 

 line near the umbones. 



Avicula costata, Sow. Tab. II, fig. 6, 6a. 



Avicula costata, Sow. Min. Con. t. 244, f. 1, 1819. 

 — — Smith. Strat. Syst., pp. 67, 81. 



— lb. Strata Ident. Clay over the Upper Oolite, f. 8. 



