18 



THE NAUTILUS. 



Ortmann, Sterki, 1 Surber, 2 Howard, 3 Lefevre and Curtis * in the 

 exceptions which these authors make for the general rule that 

 the species of the sub-family Unioninae are short-period breeders 

 (tachytictic) and those of the sub-families Anodontinae and 

 Lampsilinae are long-period breeders (bradytictic). The writer 

 agrees with Surber 2 and Howard 3 that Megalonaias heros should 

 be classed as doubtfully bradytictic. Probably it would be 

 more nearly correct to consider heros as tachytictic with the latest 

 breeding season known. Another case of eccentricity of breeding 

 season is that of Obliquaria refieoca. Although this species 

 belongs to the sub-family possessing long period of gravidity as 

 the rule, yet the writer has only found it sterile during every 

 month of the winter season. Amygdalonaias donaciformis has 

 also been found to be sterile in a few instances throughout the 

 winter months, yet this knowledge may be too meagre to estab- 

 lish it as a short-period breeder. However, being found gravid 

 with mature glochidia the early part of June may be partial 

 evidence that this species is bradytictic. The writer is able to 

 verify Surber' s discovery that donaciformis produces the smallest 

 glochidmm (0.060x0.063 mm.) on record, 2 and also that, in its 

 metamorphosis, the encysted young shows the unusual growth 

 of adult shell beyond the margin of the glochidial shell. 



In keeping this record it was occasionally noted that in a few 

 cases, especially as found in Lasmigona complanata and Lasmonos 

 fragilis, that the marsupia would contain early and late embryos, 

 and even mature glochidia, at the same time. However, this 

 irregularity is doubtless abnormal since uniformity of develop- 

 ment and maturity is the rule for all species. 



It may be of interest here to state how the author has been 

 enabled to distinguish sterile females from males of those species 

 which have no sexual dimorphism of shell and no definite dif- 

 ferentiation of marsupium and of post-ventral mantel margin 

 as seen in the Unioninae and Anodontinae. In most cases, as 



1 1903a (American Nat., xxvii, pp. 103-113). 



2 1912a (U. S. Bu. Fish., Doc. 771 ). 



3 1915 (Nautilus, xxix, pp. 4-11). 



4 1912 (U. S. En. Fish., xxx, Doc. No. 756). 



