THE NAUTILUS. 93 



fers the larger rivers, and thus no dates could be secured for the 

 spring months, yet the " interim " in July is very sharply marked. 



Lampsilis orbiculata Hildr. Autumn (Lea, III, '42). Found 

 gravid in August and September. 



Lampsilis recta Lam. Winter breeder (Sterki, '95) ; autumn 

 (Lea, '42). Found gravid in July, August, September, October. 

 No records at hand for the early summer. 



Lampsilis nasuta Say. " All the year round " (Conner, '07) ; 

 winter breeder (Sterki, '95) ; November (Lea, II, '38). I found 

 this species gravid in September (Delaware River), and on June 2 

 and 3 (in Lake Erie), when numerous gravid females were found. 



Lampsilis iris Lea. I found three gravid females on May 11. 



Lampsilis parva Barn. Winter breeder (Sterki, '95) ; May and 

 November (Lea, VII, 'GO). Extrusion of ovisacs observed by Lea 

 (ibid.) in May. 



Lampsilis (Proptera) alata Say. Winter breeder (Sterki, '95) ; 

 autumn (Lea, III, '42). Found gravid end of August, September, 

 October. 



Lampsilis (Propterd) gracilis Barn. Winter breeder (Sterki, 

 '95) ; autumn (Lea, II, '42). Found gravid in September. 



Obovaria relusa Lam. Autumn (Lea, III, '42). I found a 

 gravid female of this species on August 29, '08. 



Obovaria circulus Lea. Winter breeder (Sterki, '95) ; autumn 

 (Lea, III, 42) ; March, July-August (Lea, ibid.). I found gravid 

 females on May 27, '08. Both forms 0. circulus and lens are in- 

 cluded here : they pass into each other.) 



Obovaria ellipsis Lea. Winter breeder (Sterki, '95); autumn 

 (Lea, III, '42). 



Plagiola securis Lea. Autumn (Lea, III, '42). Gravid females 

 not rare in September and October. 



All species mentioned so far possess the " Lampsilis-type " of 

 marsupium, i. e., the posterior part of the outer gills is charged, at 

 the period of gravidity, with eggs or embryos contained in distinct 

 ovisacs. When not gravid, this part of the gills differs in structure 

 from the rest, and females are always recognizable. 



Gryptogenia irrorata Lea. Winter breeder (Sterki, '95); autumn 

 (Lea, III, '42). 



Marsupium very peculiar, but allied to the Lampsilis-type. 



Ptychobranchus phaseolus Hildr. Winter breeder (Sterki, '95); 



