PLATE 51. 
Page 
Fig. 1 & 2. Avicula tenuilamellata. 281 
1. The right valve, having the shell nearly exfoliated. 
2. The left valve of the same species, preserving some small fragment of the shell. 
Fig. 3 & 4. Avicula spinulieera. 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 sciiohari^. 288 
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 stria 1 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 manticula. 284 
7. The left valve of this species. 
8. The left valve of a smaller individual. 
Fig. 9 & 10. Avicula obliquata. 285 
9. The interior of the left valve, showing obscurely the external costa*. [ From imperfection 
in the specimen, the sinus in the external margin, between the wing and the body of the 
shell, is represented as too abrupt.] 
10. A cast of the valve, which preserves only the remains of the concentric striae. 
Fig. 11. Avicula iEQuiR adiata. 285 
11. An imperfect specimen of the left valve. 
