146 BRITISH DESMIDIACE^. 



about 27-28 granules visible at the margin. Chloro 



o o 



plasts axile, one in each semicell, with a prominent 

 central pyrenoid. 



Zygospore broadly ellipsoid, furnished with numerous 

 short, truncate or slightly emarginate processes, about 

 15 of which are visible round the periphery. 



Length 40-48 n ; breadth 25-28 /u ; breadth of 

 isthmus 11 13'5 u ; length of zygosp. with processes 

 41-43 /u, breadth 30-37 /u, length of processes 4-5 /LI. 



ENGLAND.- -Bowness and Loughrigg, Westmoreland ! 

 Puttenham Common, Surrey ! 



WALES. Capel Curig, Carnarvonshire ! 



SCOTLAND. Rhiconicn, Sutherland ! 



Geogr. Distribution. United States. 



C. isthmium is a well-marked species of very rare occurrence. 

 It is one of the western British types which are almost entirely 

 confined to the very rich places on the old formations, where 

 it is found in association with C. monomazum var. polymazum 

 and C. quadrifarium. It differs from C. excavatum Nordst. in 

 its larger size, its deeper constriction and less open sinus, and 

 in its more numerous granules. The latter are disposed in 

 vertical series, and they are generally slightly reduced in size 

 at the apices. We have examined the zygospores of this 

 species from near Orono, Maine, U.S.A. 



C. isthmium is of about the same size as C. Portianum, but 



is at once distinguished by the form of its semicells and by 



the excavated sinus, as well as by the almost circular vertical 



.view. The zygospore also differs in the possession of 



emarginate processes. 



Forma hibernica West. (PI. LXXVII, fig. 11.) 



C. isthmium forma hibernica West, Alg. W. Ireland, 1892, p. 159, t. 21, 

 f. 15 [figures very poor]. 



Cells larger, constriction less deep and isthmus 

 wider, inferior angles of semicells slightly rounded ; 

 granules not so regularly arranged, and often scattered. 



Length 50-56 /.*; breadth 31-35 /u; breadth of 

 isthmus 19-22 /n. 



IRELAND.- -Ballynahinch, Gal way ! Sugar Loaf Mt., 

 Castletown, and Grlengarriff, Kerry ! 



