94 THE NAUTILUS. 



autumn (Lea, III, '42). Found gravid in August, September, Oc- 

 tober. A specimen found on May 11, '07, had most of the ovisacs 

 discharged. None of the numerous specimens collected by the 

 writer in June and July were gravid. 



The peculiar shape of the marsupium of this species is well known. 



In the following species, belonging to Group A, the marsupium 

 occupies the whole of the outer gills, and while in Strophitus dis- 

 tinct " placentae " (Sterki) are developed, such structures or ovisacs 

 are not present in the rest. 



Strophitus undulatus Say. Autumn (Lea, III, '42); March, Octo- 

 ber (Lea, ibid.); September, December (Lea, II, '38); discharge of 

 placentae observed in January and February (Lea, X, '63). I found 

 this species gravid in the months of July, August, September, Octo- 

 ber; also in May. The latest date is May 22, '08 (one out of eleven 

 individuals). Among numerous specimens collected on May 14 and 

 May 27, '08, no gravid females were present, and during the month 

 of June such were never found, although a good number of specimens 

 were collected. The earliest date again is July 11. This gives an 

 " interim " from the end of May to about the middle of July. 



The eastern S. undidatus Say is absolutely undistinguishable fi'om 

 the so-called S. edentulus Say of the western waters. 



Anodonta caturacta Say. Breeding season, eight months during 

 the year; the interim May to October (Conner, '07); gravid in Octo- 

 ber and November (Lea, II, '38). I have seen gravid specimens 

 collected on July 23, '08, and August 21, '08. The first date, no 

 doubt, represents an exceptional case: there was only a single gravid 

 individual among forty to fifty specimens. The other date (also a 

 single individual, but only one found at that date) possibly marks 

 the beginning of the season. At any rate it is very probable that 

 the breeding season occasionally lasts longer than indicated by Con- 

 ner, and may be extended in individual cases beyond May and may 

 begin earlier than October, as is the case in other winter breeders. 



Anodonta imhecillis Say. Autumn (Lea, III, '42); March (Lea, 

 ibid.). Found gravid May 21, '08 (outlet of Lake Leboeuf, Erie 

 county), and June 2, '08 (Lake Erie). This species is hermaphro- 

 ditic, according to Sterki (Nautilus, 12, '98, p. 87). 



Anodonta grandis Say. Autumn (Lea, III, '42); July, August 

 (ibid.); October (Baker, '98). In Pennsylvania gravid females are 

 frequent in August, September, October. I have found a single 



