PLATE 53. 



rage 



Fig. 1, 4 & 6. AvicuLA COMMUNIS. 286 



1 . A young shell of this species, which preserves the anterior and posterior wings entire, with 

 the gibbous umbo and beak elevated above the hinge-line. 



4. A specimen of medium size, which is very gibbous above the middle and at the umbo. 

 Tlic wing is erroneously represented as too pointed, by leaving out a portion which is 

 obscurely visible on the posterior sinuate margin. 



6. A similar specimen, in which the body of the shell is a little more oblique than usual. The 

 posterior extension of the wing is broken off, so that the margin reijrescntcd is not the 

 natural one. 



Fig. 2, 3, 5, 7 & 10. AvicuLA textilis. 288 



2. The right valve of a young shell which is imperfect, but, in the obliquity of the body and 



general form, corresponds to this species. It is moderately convex, a little gibbous on 

 the umbo, and the surface of the shell shows subdued radii, wliilc the wing shows only 

 concentric markings as in A. communis. 



3. Tlie left valve of a young shell preserving the surface markings, with radiating striae upon 



the wing, as in the larger specimens. The posterior part of the wing is broken off, so that 

 the full extent is not shown in the figure. 



5. The interior of the left valve of a shell of this species. In the figure, the sinus on the outer 



margin is too abrupt : a cast from this interior presents precisely the characters shown 



in fig. 9, Plate lii. 

 7. The left valve of a small specimen of this species : the anterior wing and beak are broken 



off, as well as a part of the posterior wing. 

 10. A cast of the left valve of this species, from which the shell has been exfoliated and the 



posterior and basal margins broken off. The specimen is a little more abruptly convex in 



the middle and upper part than usual in shells of this species. 



Fig. 8 & 9. AVIOULA BELLULA. 289 



8. A specimen showing the two valves connected at the hinge. 



9. The interior of a left valve of the same species. 



Fig. 11 - 14. AVICUL.4. SECURIFORMIS. 290 



11. A specimen of the left valve, in which the proportions of length and height are nearly 



equal. The deep marginal sinuosity of the posterior wing is exaggerated, from imperfec- 

 tion in the specimen. 



12. The left valve, preserving the form nearly entire ; the upper part of the surface being 



exfoliated so as to obliterate the markings. 



13. The specimen is apparently the inner side of the right valve, and is quite flat. The broad 



costa: represented are the elevated spaces between the external costfe, showing as de- 

 pressions on the inside of the shell. 



14. A partial cast of the left valve of this species. 



