282 DESCEiPTiON OF {Polygaslrica. 



Clostertitm striohtum (Ehr.) — Closely but distinctly striated, 

 crescent-shaped, tapering from the middle ; sutures generally three, 

 never more ; ends very obtuse ; lower margin never inclined upwards 

 at the ends. Length l-80th to l-68th (P. 13, f. 2 and 6.) 



C. mtermedium. — Slender, sKghtly curved, tapering; strias dis- 

 tinct, not crowded ; sutures usually more than three ; ends truncate. 

 Its most remarkable feature is the number of its sutures, whicli 

 exceeds that of any other species. Length l-77th to l-54th. 



C. angustatum. — Sublinear, curved, scarcely attenuated ; ends 

 truncate ; strise few, very distinct and prominent ; sutures usually 

 three. Vesicles in a single row ; fillets obscure. Length 1-60 th. 



C. juncidtim. — Very slender, linear, straight, except the extremi- 

 ties, which are slightly attenuated and curved downwards. 



Var /'b.J — Stouter, and less elongated ; its colour, when empty, 

 deeper, and its strise more conspicuous. Length of {'a J 1- 111th to 

 l-69th. Length of ('bj l-144th. 



2. Frond striated, much elongated, gradually tapering, scarcely ros- 

 trate ; sporangium hiloied, situated between the conjugated fronds. 



C. lineatum (Ehr.) — Stiiated, slender, long, curved, gradually 

 tapering into the conico-rostrate extremities ; lower margin slightly 

 inflated at its centre ; fillets three or more, frequently obscure ; vesi- 

 cles in a single row ; one or more transverse lines at the centre ; 

 longitudinal striae numerous. 



Var. (b.) — Longitudinal striae numerous. 



C. lineatum connects the rostrate with the other striated species, 

 as in general aspect it agrees with some of the latter, and in its 

 inflated centre and tapering extremities with the former. 



The fronds approach and couple in the usual manner ; but instead 

 of the contents of both fronds uniting in the ordinary manner into 

 a single body, a bilobed body is produced, not unlike a species of 

 Cosmarium. Though called bilobed, Mr. Ralfs regards the sporan- 

 gium as bitiate, being readily separable at the constricted line of 

 junction, (page 279.) Length 1 -48th. 



3. Frond either rostrate or minute, colourless and acicular; sporan- 

 gium cruciform. { Stauroceras, Kutzing.J 



I Frond striated, tapering at each end into a distinct beak. 



C. lialfsii. — Stout, striated, curved, rapidly attenuated into linear 



