118 FEESH-WATEE EHIZOPODS OF XOETH AMEEICA. 



crenulate. Sarcode colorless; pseudopods many, of the usual form in the 

 genus. 



Size. — Eanges from 0.14 mm. to 0.32 mm. in diameter; mouth 0.06 

 mm. to 0.18 mm. wide; spines 0.04 mm. to 0.06 mm. long. 



Locality. — Ditches near Philadelphia, ponds on Darby Creek, and 

 Swarthmore brick-pond, Delaware County, Morrisville pond, Berks 

 County, Pennsylvania; Atco pond, Woodstown pond, Hammonton pond, 

 Absecom pond, and Lake Hattacawanna, New Jersey; Spencer pond, 

 Maine; Jacksonville, Florida. England, Wallich. 



Diflliigin corona, as represented in the figures of pi. XVII, is the 



most remarkable and beautiful species of the genus. It was first indicated 

 by Dr. Wallich from specimens found in England It is a common Ameri- 

 can species, and is frequent in the vicinity of Philadelphia. 



The shell of Difflugia corona is usually nearly spherical, but frequently 

 is slightly prolonged approaching the mouth. It is commonly one of the 

 smoothest of the genus, though composed of angular particles of quartz- 

 sand in the usual manner. The mouth is circular, and commonly trun- 

 cates the spheroidal shell, or it continues its curvature, or it is somewhat 

 projected. The lip or border of the mouth is dentated or crenulated. 

 The denticles are thick, angular processes, more or less acute, sometimes 

 blunted, concentric, or slightly everted. They range in number from six 

 to sixteen; but the most frequent number is twelve, and usually a larger 

 rather than a smaller number prevails. The intervening notches are thick- 

 edged, as deep as they are wide, and rounded at bottom. 



The fundus of the shell is furnished with a variable number of acute 

 conical spines. Generally there are from three to seven ; but they range in 

 number from one to eleven. Mostly they form an eccentric circle, widely 

 divergent, nearly equidistant, and usually occupy a position at the upper 

 third of the shell. Often there is a central spine, sometimes longer than 

 the others. Often this is absent, and sometimes it is the only one existing. 

 The spines are straight or slightly curved, sharp-pointed, hollow processes 

 of the shell, with the same composition. 



The shell is composed of colorless angular quartz-sand, usually with 

 the larger particles scattered, with some appearance of uniformity, and 

 with the intervals occupied by smaller ones. Often, too, larger stones are 



