for tJie Microscope. 25 



This is rather an elongated form, being twice as long as 

 broad, deeply constricted into two equal segments, each 

 of which is somewhat triangular, with rounded angles. 

 Not uncommon. 



180. MINUTE COSMARIUM (Cosmarium bioculatuni). 

 This very small species is divided on each side by a gap- 

 ing notch into two elliptical segments, having the broad- 

 est sides united by a distinct neck. Its minute size occa- 

 sions it to be often overlooked. 



181. ANGULAR COSMARIUM (Cosmarium botrytis). 

 This species differs from C. margaritiferum in its more 

 elongated form and flattened ends, so that each lobe is 

 more triangular than kidney-shaped. The beaded margin 

 will serve to distinguish it from the " pyramid cosma- 

 rium." 



182. BEAUTIFUL COSMARIUM (Cosmarium ornatum). 

 Smaller than the last, from which it is distinguished by its 

 less regular outline, and its end view, instead of being 

 elliptical, has a convex projection on each side, so as to 

 be cross-shaped. (PI. III., fig. 13.) 



183. GOURD COSMARIUM (Cosmarium cucurbita). The 

 surface is not granular, as in the preceding. It is a small 

 species, with nearly straight sides and rounded ends. The 

 cud view is circular. This is by no means an uncommon 

 species. (PI. III., fig. 14.) 



184. FORK-SPINED XANTHIDIUM (Xantliidium arma- 

 tum}. This is like a species of Cosmarium, having a spiny 

 margin. The segments are almost kidney-shaped, and 

 fringed with short spines, each of which is divided at the 

 apex into three or four points. 



185. SHARP-SPINED XANTHIDIUM (Xanthidium fasdcu- 

 latuni). The segments are usually kidney-shaped, as in 

 the last species, but the spines are slender and pointed, in 

 pairs at the margin; each segment has commonly four 

 pairs of spines. 



1 86. HORNED STAURASTRUM (Staiirastrum dejcctum}. 

 In the side view this little species consists of two elliptic 

 or half-moon-shaped segments, with each horn or extre- 

 mity ending in a sharp spur. The end view shows three 



