258 DESCRIPTION OF \JPolygastrica. 



XAJfTHiDrtTM fascicidatum (Ehr.) — Segments with foiir to six pairs of 

 subulate marginal spines; central projections minute, conical, not 

 beaded. 



Var. {a.) — Each segment with four pairs of spines. 



Var. (J.) — Six pairs of spiues to each segment. Length 1 -454th. 

 to l-350th. 



X. cristatmn. — Segments with a solitary spine on each side at the 

 base; the other spiaes gemiuate, (P. 13. f. 18 and 23, the latter an 

 end view.) 



Var. {a.) — Segments reniform ; spines scarcely curved. 



Var. {I.) — Segments truncate at the end; spiaes uncinate. Length 

 l-357th. 



X. (?) octocorne (Ehr.) also = Arthrodesnms octocornis (Ehr.) — Seg- 

 ments much compressed, without a central protuberance, trapezoid ; 

 each angle terminated by one or two spines. Erond minute ; iuterval 

 between the angles concave. 



Var. {a.) — One spine at each angle. 



Var. (1.) major. — Larger, two or more spines at each angle. 



The proper position (says Mr. Kalfs) of this plant, wantiug, as it 

 does, the central protuberance, is very doubtful. Ehrenberg placed 

 it first in Arthrodesynus, and afterwards in Xanthidium. Length 

 l-1351stto l-1020th. Common. 



X. Artiscon (Ehr.) — Segments naiTowed at the base ; and margin 

 with numerous elongated spiaes, which arc divided at the apex into 

 three lobes. Figured by Dr. Bailey. 



It differs from X. armatum by its segments tapering at the base ; 

 its spines also are much longer and are more restricted to the outer 

 and rounded margin. Diameter 1-11 52nd to 1 -288th. 



'K. fHrcatum (Ehr.) — Corpuscles globose, green, single or binate, 

 spinous; spines scattered, forked at the apex. Size 1-5 70th to 

 l-280th (fig. 110.) 



This is probably a Staurasfrum. According to Ehrenberg the 

 corpuscles sometimes occur in fours ; also some have but one spine, 

 and some short and broad processes. The spines are usually half the 

 length of the body. 



Under the name Xanthidium, Ehrenberg described various bodies, 

 more or less branched and orbicular (P. 12. f. 511 to 515) found in 



