56 TESTACEA. 



farther investigation, and which will not be difficult, from the facility of 

 obtaining specimens, and being of the season of propagation. 



It is uncertain how and why the swelling and diffusion of the spawn 

 succeed, whether by the generation of elastic fluids to exert or maintain 

 the respiratory functions of the nascent being. 



This species seems the natural prey of some of the leech tribe, espe- 

 cially the Hirudo bin-oculata. Neither does it escape the irresistible at- 

 tack of the Hirudo complanata or six eyed leech. Numerous empty 

 shells are confined along with living specimens between the lower parts 

 of the leaves of the Iris, where also is the lurking place of their insidious 

 enemies. 



PLATE XI. 



FIG. 11. Valvata obtum. 



12. Shell, slightly enlarged. 

 IS. Specimen, upper surface. 



14. Under surface. 



15. Upper surface ; operculum a. 



16. Branchial organ. 



The preceding figures 13-17 enlarged. 



17. Spawn. 



18. The same, enlarged. 



19. Spawn, enlarged. 



20. Capsule, empty. 



21. Diffusing spawn. 



22. Portion of the same enlarged, shewing the dispersion of the embryos. 



23. Embryos moving within their respective integuments. 



24. Empty shells of nascent animals. 



