270 DESCEIPTION OF {Polygastrica. 



but subsequently (1840) two other species were added, which, from 

 the given characters, we presume to be Staurastra, but shall describe 

 them in Ehrcnberg's own words. 



PEJfTASTEEiAS oUusa. — Corpusclcs solitary, sub-orbicular, penta- 

 gonal, green ; laterally, ovate oblong, smooth. Diameter 1-1 150th. 



P. radiata. — Corpuscles in pairs, with elongated rough rays 

 equalling their diameter, rays connivent ; ovarium green, in five 

 segments (quinquefid). Entii'e diameter l-860th. 



Genus DiDYMocLAnoif (Ealfs.) — Frond simple, constricted at the 

 middle, angular, each angle having two processes, one lateral, and, 

 in front view, nearly parallel to the adjacent one of the other seg- 

 ment, the other superior and divergent. 



This genus is closely allied to Staurastrum, but differs by each 

 angle of the segments giving rise to two processes, one beneath the 

 other; whereas, in Staurastrum, the process, when present, is 

 solitary. 



D. furcigerus. 



Frond comparatively large, rough, with pearly granules, which, 

 being arranged on the processes in tranverse Irues, produce a crenate 

 appearance on their margins. In the front -view, the processes are 

 elongated, stout, tapering, bifid at the apex. Length, exclusive of 

 processes, l-547th. Breadth l-555th. 



Var. («.) — End view triangular. 



Var. {b.) — End view quadi-angular. 



^^ '^W5^ 



Front view. End view. 



D. cerherm (Bailey). — Small, deeply constricted, segments three- 

 lobed ; lobes with four teeth, two of which project. Lakes in 

 Florida. 



D. longnpinum (J^qxIgj .) — Large, smooth, triangular, with two long 

 ppines at each angle. Lakes in Florida. 



The two last species are not veiy nearly allied to each other, nor 



