342 Dr. PiTTON on the Strata below the Chalk. [App. A. 



Fig. 4. Venusi suhmersa. Nearly orbicular ; valves tumid ; the posterior extremity truncated ; 

 lunette obscure ; hinge-slope rather straight. 



Fig. 5. Venusi suhlcevis. Very flat, elliptical, smooth ; lunette undefined, not sunk. 



Fig. 6. Venusi immersa. Very flat, ovate, smooth ; lunette deeply sunk; its edge not defined. 



Fig. 7. Cucutlcea formosa. More deeply striated than C. fibrosa; and in form resembling 

 C. carinata (Min. Conch, t. 207.). Very convex, transversely elongated; posterior extremity 

 pointed. 



Fig. 8. ^rca rotundata. Surface longitudinally striated ; the sides unequal ; both rounded. 

 There is no space between the beaks. 



Fig. 9. Nucula lineata. Elliptical ; posterior extremity slightly truncated, with a short point 

 at its upper angle ; surface transversely striated, with lines straighter than the lines of growth, 

 which they consequently cross twice. 



Fig. 10. Nucula apiculata. Convex, smooth, transversely obovate; posterior extremity pointed. 



Fig. 11. Nucula obtusa. Convex, smooth, elliptical ; lunette prominent, elongated. 



Fig. 12. Trigonia quadrata. Nearly square, fiat ; anterior extremity rounded ; concentrically 

 ribbed ; each rib bent at a right angle in the middle, where there is an obtuse knob. 



Fig. 13. Modiola reversa. Transversely elongated, rather flattened; the posterior portion 

 expanded ; surface ornamented with thin concentric ridges, which are bent back or reversed 

 upon the shell. In some individuals they are worn away. 



Fig. 14. Mytilus tr'idens. The hinge is furnished with three unequal teeth ; elongated, convex, 

 carinated ; beaks pointed ; the surface very smooth. The teeth distinguish this species from 

 M. edentulus (Min. Conch, t. 439. fig. 1.). 



Fig. 15. Mytilus prcelongus. Hinge without teeth; shell very thick, much elongated, keel- 

 shaped. It approaches to M. lanceolatus (Min. Conch, t. 439. fig. 2.) ; but, independently of its 

 remarkable thickness, the sides are nearly parallel, as far as the termination of the hinge-line, 

 which is marked by an angle. 



Fig. 16. Mytilus incequivalvis. Shell broad; one valve much flatter than the other; both 

 smooth. 



Fig. 17. Perna roslrata. Ovate, nearly flat; the surface very smooth ; the lesser wing pro- 

 duced ; the shell thin, contracted at its base. Fragments of this species are very common at 

 Blackdown, but perfect specimens rare. 



Fig. 18. Avicula anomala. Obliquely elongated, imperfectly five-angled, flat along the middle ; 

 many longitudinal, narrow, elevated ridges extend over the surface, and are crossed by fine lines 

 of growth. The valves are very deep, together measuring about l| inch, with a square section; 

 length three inches. This shell I believe to be an Avicula, although the hinge is not visible. 



Fig. 19. Pecten Millerii. Oblong, rather convex ; radii smooth, sharp, numerous, especially 

 towards the edge ; close together. The two smaller figures represent an unusually convex 

 specimen. 



Fig. 20. Pecten compositus. Oblong ; with about 20 smooth, sharp, radii, and two rows of 

 scales between each of them. 



Fig. 21. Limal suboimlis. Rather quadrangular, elongated; radii very numerous, equal to 

 the furrows between them, and ornamented with one row of rather obtuse distant scales. 



PLATE XVIII. 



Fig. 1. Pecten Stutchburiensis. Suborbicular, compressed; radii more than 60, alternately 

 smaller, scaly ; those towards one side large and distant, with oblique striae between them. 



