1909.] Hermaphroditism in Mollusc Crepidula fornicata. 483 



that protandric hermaphroditism may be found to be much more common in 

 the Streptoneura than is thought at present. Pelseneer mentions 10 other 

 Streptoneurous hermaphrodites, one of which, Entoconclia, is known to 

 be protandric (6, p. 159); three others, Entocolax, Entosiphon (7) , and 

 Ezterozenos (8), are probably protandric. Six others occur, Valvata, 

 Bathysciadium, Odostomia, Coccidina, Oncidiopsis, Marsenina, of which I have 

 not found descriptions. C. fornicata may now be added to this list. 

 Hence, it would appear that one of the chief distinctions between the 

 Streptoneura and the Euthyneura is beginning to break down. 



The sex phenomena observed in Crepidula, fornicata support in a striking 

 manner G. Smith's view (4, pp. 88, 89) of sedentarily-induced hermaphroditism, 

 that is, suppression of females ; moreover, the genus may be reasonably 

 expected to offer stages in the evolution of this hermaphroditism, and so 

 afford a means of testing the above-mentioned view. In the early stages of 

 its evolution we should expect to find : — 



1. A small percentage of young females among the spat. 



2. Adult females of two kinds — 



(«) Those born as females. 

 (h) Those born as males. 



Eecent Researches on G-ametogenesis — besides the known fact that some 

 Tsenioglossa have two kinds of spermatozoa (6, p. 125) — give some hope that 

 the two latter categories might be distinguishable by the cytological 

 characters of their gametes. 



Summary. 



Crepidula fornicata is a Streptoneur of the family Calyptrseidae. 



Individuals of this species associate permanently in linear series to form 

 " chains." All lengths of chain composed of upwards to as many as 12 

 individuals have been found. 



All the young are able to creep about, but the adults are sedentary. 



The individuals in a chain offer a transitional series from maleness to 

 femaleness both in primary and secondary sexual characters. Since all the 

 young ones are males, the species is a protandric hermaphrodite. 



Dwarf females occur as " physiological varieties." 



Allied species and a species of an allied genus will very likely be shown to 

 be protandric hermaphrodites. 



There is good reason for thinking that this sex phenomenon may be even 

 more widely spread in the Streptoneura. 



Since the males in this species change into females, it would seem in this 



