TESTACEA. 31 



they next become more distinct, as on July 7, fig. 20. Farther, by ad- 

 vancing increment, an elongated white shell occupies the whole space 

 within the integumentary outline, as on July 17, fig. 21. 



The young attain maturity in about a month, or within a few days 

 less or more. They are very minute, pure white, and, to the eye, of a 

 long oval shape. But under the microscope, they prove of a true regular 

 oval ; the animal greyish-brown, with its two black eyes distinct ; and 

 then there is evident pulsation of the heart. The margin of the shell at 

 that time light-grey, spreads an oval border around its contents. 



Here, it may be observed, that the figures of originating parts is 

 affected by their transparence. A narrower or a broader oval on a re- 

 gular circle, was determined more justly by the microscope than by the 

 unarmed eye. 



These animals have bred from the beginning of June to the begin- 

 ning of August. 



Tliey are not so common as the preceding species. 



Plate VI. 



Fig. 1. Ancylm flaviatilis, shell, upper surface. 



2. Animal in the shell, under surface, outline. 

 S. Animal in the shell, slightly enlarged. 



4. Another specimen, under surface. 



5. Spawn consisting of two ova, September 27. 



6. The same, enlarged. 



7. The same, farther advanced, October 20. 



8. 9, 10. Spawn with ova in various number and arrangement. 



11. Enlarged view of fig. 8. 



12. Spawn of August 23. 



13. The same, enlarged. 



14. Nascent Ancylus from the preceding spawn, back of the shell, en- 



larged. 



15. Exterior of the empty shell of an Ancylus, enlarged, 



1 6. The same, more enlarged. 



17. Adult regenerating the inner part of the shell in front, enlarged. 



18. Ancylus lacustris, upper surface. 



19. The same, under surface. 



20. Spawn, as on July 7, enlarged. 



21. The same, as on July 17, enlarged. 



