256 DESCEiPTiON OF [Poli/ffastrica ^ 



curtum, in its differently arranged endochrome ; and from t'. turgidum 

 and C. attenuatiim, in its nearly parallel sides. (l-357th.) 



CosMAnruM curtum. — Minute, smooth; oblong; constricted; rounded 

 at the ends ; cndochrome in longitudinal fillets. End view circular. 

 (l-465th.) 



C. attemiafum. — Fusiform, three or four times longer than broad ; 

 slightly constiicted ; ends obtuse. It is doubtful, says Mr. Ralfs, 

 "whether this and C. turgidum ought not to be removed to Pe)iium. 



C. turgidum, — ^Large, turgid, oblong, punctate, constricted ends 

 broadly rounded. It differs from the allied species in its much larger 

 size. It is comparatively longer and more tapering than C cucurhita 

 and C Thicaitesii, and its ends are more rounded than those of C. 

 attmuatum. Length l-126th. 



C. lagenarimn, (Corda). — Segments triangular; all the angles 

 broadly rounded. 



C- ventricoswn (Kiitz.) — Mr. Ralfs says, "I believe this to be a 

 species of Cosmarium in a dividing state." 



Genus XANTHiniFM, (Ehr.) — Simple, constricted in the middle ; 

 segments comj)ressed, entu-e, spinous, having a circular projection 

 near the centre, which is usually tuberculated. Segments reniform, 

 orbiculai-, or angular ; sj)ines scattered or arranged in two rows, one 

 on each side of the marginal line. 



The resemblance of Xanthidium^ and Staurastrmn is more apparent 

 than real, the cells of the latter are angular, UTespective of the pro- 

 cesses. In Arthrodesmus, each segment has only two spines, one on 

 each side, and there are no central projections. The connection with 

 Cosmarium is far more intimate ; in fact, the sole distinctive character 

 that can be relied iipon, is the presence of spines in this genus. 



Several plants referred by Ehrenberg to Xanthidium, are angular 

 in the end view, and properly belong to Staurastrmn, thus : X hir- 

 iutum = Staurastrmn hirsutum, (EaLfs.) 



Agassiz believes the Xanthidia, generally to be the sporangia of 

 Algce; but he adduces but one instance in favour of this notion, 

 ■where he observed Xanthidia, growing as Sporangia, uj)on the 

 branches of an Alga), surrounded with mucus. 



Ehrenberg' s chaxactcrs of the genus Xanthidium or double-bur 



