20 



STUDIES IN EVOLUTION 



dissoconch is at first generally smooth, save for the fine con- 

 centric lines of growth (figure 25). In ornamented or spinose 

 species, however, irregularities in the growth lines soon appear 

 (fio-ures 26, 27), and these shortly assume the characteristic 

 surface sculpture of the normal adult. Thus the prodisso- 

 conch of Avicula sterna is represented at p, figure 25, and 

 is followed by regular concentric growth during the nepionic 



24 



25 



26 



Figure 24. — Prodissoconch of Ostrea virginiana. X 43. 



Figure 25. — Each stage of Avicula sterna; p, prodissoconch. X 19. 



Figure 26. — Young Avicula sterna ; showing the beginning of epine growth. 

 X 3. 



Figure 27. — Young Saxicava arctica. X 19- 



Figure 28. — Young Anomia aculeata; prodissoconch succeeded by early 

 smooth and later spinous dissoconch growth. X 30. (Figures 24-28 after 

 Jackson.) 



stages. In figure 26 the spiny characters of early adoles- 

 cence are added to the previous stages, and in later stages 

 the spines become more and more emphatic. 



In Spondylus the prodissoconch is the same simple form, 

 and is succeeded by a nearly smooth Pecten-like stage, dur- 

 ing which the animal was free (figure 29). After fixation 

 the growth is very irregular and ostrseiforra for a time, until 

 the shell rises above the object of support, when all the most 



