GENUS DIFFLUGIA— DUTLUGIA PYEIFOEMIS. 101 



In the summer of 1874, I found this variety in extraordinary profusion 

 in Swarthmore brick-pond, and all the specimens of pi. XI were derived 

 from this locality. The subsequent two seasons I could find none of the 

 kind in the same pond, but in 1877 it again appeared in moderate quantity. 



When first discovered, the specimens were so abxmdant that a drop of 

 ooze, in which the animal lived, would often contain several dozen indi- 

 viduals. 



They were remarkable for their large size, and the brig) it green color 

 and activity of the animal. Notwithstanding the great number of indi- 

 viduals, scarcely two could be found in all respects alike. The general 

 shape was that of the ordinary D pyriformis, usually more or less com- 

 pressed ; but the proportions varied considerably. Commonly the breadth 

 was nearly or quite double in one direction what it was in the other ; but 

 sometimes the compression was trifling, when the shell presented the 

 ordinary form of B. pyriformis. Commonly the greatest breadth occupied 

 the upper third or fourth of the shell. 



In the view of the specimens from the narrower side, the shape was 

 pretty uni^rmly pyriform. In the view from the broader side, the lateral 

 border varied from an inclined plane to a concave line, while the fundus 

 varied exceedingly ; sometimes from evenly convex to obtusely angular ; 

 sometimes from horizontally straight to concave ; sometimes with a single 

 median eminence, a pair of lateral eminences, or a transverse row of three, 

 and all variable in their proportions. Unsymmetrical forms also occurred, 

 in which the knobs or eminences on one side of the shell were dispropor- 

 tionately produced. 



The size of the knobby specimens, from Swarthmore pond, ranged as 

 follows : the smallest specimen was ^d of an inch long ; ith of an inch in 

 the greater, and ji^th of an inch in the lesser breadth. A large specimen 

 measured ^th of an inch long ; ^th of an inch in the greater, and ^th of 

 an inch in the 'lesser breadth, with the oral end jith of an inch wide. 



In my early observations on the variety D. nodosa, T was led to view 

 it as a species distinct from Z>. pyriformis, and from the bright green color 

 of the endosarc I named it D. entochloris. 



Another variety of Bifflugia pyriformis, which I formerly supposed to 

 be a distinct species, I named B. vas. It has the shape of the more charac- 

 teristic sjoecimens of B pyriformis, but has the neck defined from the body 



