verv slightly inflated so that both sides and apex of 

 srniicelis are a little convex; cell-wall thinner; spines 

 considerably shorter and not so robust. 



Zygospore subglobose, furnished with simple, acnte 

 spines. 



Length without spines 12'5-16'5 p., with spines 

 10-25 /LI; breadth without spines 12'5-13*5 p., with 

 spines 19-26//,; length of spines 4-9 /x,; breadth of 

 isthmus 5*5-7 JJL ; thickness 7-8' o p. ; diam. zygosp. 

 without spines 19ft, with spines 33-37 fi. 



ENGLAND.- -Cumberland ! Westmoreland (zygospores 

 from several localities) ! W. and X. Yorks. (zygo- 

 spores from near Halifax and in bog 2 miles S.W. 

 of Clapham) ! Cheshire (Ron). Leicestershire (Roij). 

 Warwick! (Wills). GTloucester ! Surrey (zygospores 

 from Chobham and Esher Commons) ! Sussex ! 

 Hants ! Devon ! Cornwall ! 



WALES.- -General (zygospores frequent in Carnar- 

 vonshire and Merioneth) ! 



SOOT LAND.- -General ! (Roy $ Bissett). Frequently 

 conjugated! Lewis, Harris, N". and S. Uist, and 

 Benbecula, Outer Hebrides ! Orkneys ! Shetlands ! 

 Frequent in the plankton ! 



IRELAND. Very frequent (zygospores not un- 

 common) ! 



Geoyr. Distribution. --Consult that under the type 

 form, which it has been impossible to unravel. 



This small form is perhaps one of the most abundant forms 

 of Artlirodesmus Incus met with in the British Islands, and is 

 certainly the most frequent one observed with zygospores. 

 It is sufficiently distinct by reason of its small size and 

 more delicate spines. It is found principally amongst 

 Sphagnum, both in bogs and pools. 



Forma perforata Schmidle. (PL CXIV, fig. 1 .) 



Arthrodesmus Incus forma perforata Schmidle, Lappmark Siisswasser- 

 algen, 1898, p. 42, t. 2, f. 11. 



Spines shorter; cell- wall with a few scattered, 

 irregularly disposed scrobiculations. 



