ST AURA STRUM. 65 



pools. Xanthidiiim aculeatum has the spines 

 simple, shaped like a horn, and numerous ; 

 other species have them longer, and but few in 

 number. 



The genus Arthrodesmm has the segments 

 entire, with a single spine on each side; the 

 two segments, joined by a very narrow band, 

 are broader than long, and smooth. There are 

 often four united together. Arthrodesmm con- 

 vergens (Drop I. fig. 6) has the spines of its seg- 

 ments curved towards each other. A. incus has 

 the spines diverging outwardly, and the ends 

 truncated. 



The genus Staurastrum has a great variety of 

 species, amounting to about forty. They are 

 mostly of minute size, and differ but slightly at 



