20 CONKLIN. [Vol. XIII. 



in each of these species shows that this is probably the case, 

 for associated with these differences in the number of ova pro- 

 duced are profound differences in the later stages of develop- 

 ment. C. plana and C. fornicata pass through a long larval or 

 veliger period, but C. convexa and C. adunca have no free- 

 swimming larval stage at all, the young crawling directly out of 

 the &g^ capsules in a condition practically adult. Since vast 

 numbers of the free-swimming veligers of C. plana and C. for- 

 nicata, must be destroyed before reaching that stage of devel- 

 opment at which the young of C. convexa and C. adunca first 

 issue from the egg capsules, it is evident that vastly more eggs 

 must be produced by the two former species than by the latter 

 if these different species are to continue in the same relative 

 numbers in which they are now found. 



Correlated with the different number of ova produced by the 

 three species are noteworthy differences in the size of the ova, 

 as is shown by the following tables and diagram : 



Table II. 



Absolute Measurements of the Unsegmented Eggs of 

 Crepidula. (Approximate.) 



(All measurements were made on eggs preserved in alcohol and mounted in 

 Canada balsam.) 



Species. Diameter. Volume. Number of 



Eggs Laid. 



C. plana (type), .136 mm. .00131709 cu. mm. 9000 



C. plana (dwarf), .136 " .00131709 " " 3070 



C. fornicata, .182 " .00315655 " " 13200 



C. convexa, .280 " .01149406 " " 220 



C. adunca, .410 " .03608703 " " 180 



Table III. 



Relative Measurements of the Unsegmented Eggs of 

 Crepidula. (Approximate.) 



Species. Diameter. Volume. Number of Eggs Laid. 



C. plana (type), i i ^q 



C. plana (dwarf), i i j- 



C. fornicata, i^ 2f ti 



C. convexa, 2 8| li 



C. adunca, 3 274 i 



