PLATE 51. 



Pace 

 Fig. 1 & 2. AVICULA TENUILAMELLATA. 281 



1. The right valve, having the shell nearly exfoliated. 



2. The left valve of the same species, preserving sonic small fragment of the shell. 



Fig. 3 & 4. AvICULA SPINULIPEEA. 282 



3. The interior of the right valve, from which the posterior wing is nearly removed. 



4. The impression or mould of the exterior surface of the left valve of the same species. 



Fig. 5. AvicuLA SCHOHARIE. 283 



5. Left valve of this species. The line apparently corresponding to the anterior wing in the 



the figure is an accidental ridge in the stone. 

 5 a. Enlargement of a portion of the surface, to show the round radiating striaj and distant 

 lamellose lines of growth. 



Fig. 6. AviCULA UMBONATA. 284 



6. The left valve, having the posterior wing broken off. 



Fig. 7 & 8. AviCULA MANTICITLA. 284 



7. The left valve of this species. 



8. The left valve of a smaller individual. 



Fig. 9 & 10. AviCULA OBLIQUATA. 28.5 



9. The interior of the left valve, showing obscurely the external costas. [ From imperfection 



in the specimen, the sinus in the external margin, between the wing and tlie body of the 

 shell, is represented as too abrupt.] 



to. A cast of the valve, which preserves only the remains of the concentric striae. 



Fig. 11. AviCULA .lEQUIRADIATA. 285 



11. An imperfect specimen of th3 left valve. 



