23 



1. aculeata, Gmel. 



2. adunca, Soto. 



3. Ajilysioides, Reeve. 



4. arcuata, Brod. 



5. bilobata, Gray. 



6. convexa, Say. 



7. costata, Desk. 



8. dilatata, Lam. 



9. excavata, Brod. 

 10. exuviata, Nutt. 



Species. 



11. fimbriata, Reeve. 



12. foliacea, Tfrot?. 



13. fornicata, Linn. 



14. glauca, /Say. 



15. hepatica, Desk. 



16. incurva, 5roJ. 



17. Lessonii, id. 



18. lirata, Reeve. 



19. marginalia, _Z?ro^. 



20. Nautiloides, Less. 



21. onyx, /Sow. 



22. pallida, Brod. 



23. porcellana, Linn. 



24. rostrata, C. i?. Adams. 



25. rugosa, iV?^. 



26. scabies, Reeve. 



27. squama, Z?/*o^. 



28. unguiformis, Lam. 



29. Walshi, Herm. 



Figures. 



Crepidula costata. PL 23. Fig. 135. Shell of a finely-developed ribbed 

 species, showing the internal appendage modified into a cross deck. 



Family 4. FISSURACEA. 



Shell ; dish-shaped or conical, not spiral, perforated or notched 

 or siphonated. 



In dividing the class of Gastropods into Orders according to the struc- 

 ture and position of the branchiae, the arrangement up to this point is na- 

 tural enough ; but now the systematist finds himself a little perplexed. 

 " It would seem/' says Prof. Forbes, " to be a law, in both animal and 

 vegetable kingdoms, that no character, whether of structure or form, pre- 

 serves an equal value in every tribe, but varies in its importance ; in one 

 group characterizing a class, in another scarcely determining a species." 

 The several kinds of Limpets which come into our present Order, Cervico- 

 hranchiata, on account of the branchiae being contained within a cavity of 

 the neck, include a somewhat miscellaneous assemblage of genera, as much 

 distinguished from one another in many important particulars as they are 

 from the Patellce, or true Limpets, which are included, after the method of 

 Cuvier, in a separate Order along with the Chitons, on account of the 

 branchiae being placed around the body beneath the edge of the mantle. 



The Fissuracea, so named after the principal characteristic of the group, 

 have their shells either perforated at the apex or in front, or at the front 

 margin ; and to the fissured Limpets must be temporarily referred the 

 genus Siphonaria, of which the shells exhibit the mark of a siphon, as well 

 as Acmaa and Gadinia, of which the shells cannot well be distinguished 



