274 DESCEIPTION OF \^Poli/gastrica. 



gated, linear ; consti'ictcd. at the middle ; tiamcate at the ends ; seg- 

 ments usually inflated at the base. A suture is generally very dis- 

 tinct, in a line "with the constriction. This genus, like Closterium, 

 has terminal globules containing moving granules, and its vesicles 

 are either scattered or an-anged in a single longitudinal row. It 

 differs from Closterium in its straight fi-onds and constricted middle ; 

 and from Penium, it may be known by having a distinct constriction 

 at the middle, and more elongated fronds. 



DocrDiUM iiodulosiim. — Large, stout, segments foiu* to six times longer 

 than broad, constricted at regular intervals, so as to produce undulated 

 margins ; sutui'e projecting on each side. D. nodulosum difi'ers from 

 D. constrictum and D. nodosum, in its more numerous but slighter 

 constrictions. 



D. truncatum. — Large, stout ; sutiu^e projecting on each side. Its 

 distinctive marks are, the more attenuated extremities and the soli- 

 tary inflation at the base of each segment, so that its mai'gins are 

 not imdulated. Length l-81st to l-72nd. 



D. cla/vatum. — Segments many times longer than broad, with a 

 single inflation at the base ; ends clavate, but truncate ; suture not 

 projecting. Vesicles small, few, and arranged in a single longitudinal 

 line. Empty frond minutely punctate. Length l-65th (P. 13. f. 9.) 



D. Ehrenlergii ^= Closterium Trabecula (Ehr.) — Segments elon- 

 gated, with two or three slight inflations at the base ; ends truncate, 

 bordered by minute tubercles (P. 13. f. 8 and 11.) 



Var. (l.J — Erond rough, with minute granules. Frond as long 

 as those of D. noduhsum and D. truncatum, but far more slender ; 

 constricted at the middle; suture strongly marked but not projecting. 



D. haculum. — Segments very slender, having a solitaiy and con- 

 spicuous inflation at the base, otherwise linear ; vesicles in a single 

 series. Suture strongly marked, not projecting. Length 1-1 11th. 



D. minutum. — Small ; slender, slightly constricted at the middle ; 

 segments lineax-, not inflated; vesicles iu a siagle series. Length 

 l-212th. 



D. ? asperum. — Slender, twelve or more times longer than broad, 

 cylindrical, rough ; Mr. Ralfs has seen neither vesicles nor terminal 

 globules. Unlike all the true JJocidia, this has neither constriction 



