270 MESSES. W. WEST AND G. S. WEST ON 



are three warts (transversely disposed) : the outer ones are truncate or subemarginate,and 

 are each situated under a spine, there being two spines at each angle; the central one, 

 which is also situated under a spine in the typical form, is deeply emarginate in a vertical 

 direction, and the lower portion of it sometimes bears a short downwardly-directed spine. 

 Thus, when seen in vertical view, each angle is furnished with three subtruncate 

 "prominences." This is so in the typical forai and in var. brachyacanthum also, but 

 Wolle says that v. simplex has but two of these. This is certainly remarkable, seeing 

 that these two varieties have the same number of spines. The "' one-sided, hastate, 

 poignard-like spines " are due to the spines being set on rather to the inner side of short 

 ascending processes of the Staurastrum. 



167. Staurastrum verrucosum, nov. sp. (PI. XVIII. fig. 10.) S. submediocre, paulo 

 longius quam latius, modice constrictum, sinu parvo subacute; semicellulaB obtra- 

 peziformes, angulis inferioribus inflatis et annulis denticulorum parvorum (circiter 8 

 in ambitu), lateribus nodulis acutis (interdum subobtusis) tribus, angulis superioribus 

 subtruncatis, apicibus vcriucis magnis emarginato-truncatis 4 obsessis, intra angulos 

 superiores granulis subirregulariter dispositis ; a vertice visas triangulares, lateribus 

 concavis et undulato-denticulatis, angulis truncatis, verrucis binis in seriebus tribus 

 transverse dispositis intra angulum unumquemque, medium versus cellulae majoribus. 



Long. 35 fx ; lat. 29 fx ; lat. istlim. 9*5 ^. 



Genus Dichotomum, nov. ^Qn. 



CellulsB modice constricta3, semicellulaB bilobao, lobis dichotomis ; a vertice visae anguste 

 fusiformes. 



168. D. ELEGANs, nov. sp. (PI. XVI. fig. 33.) D. mediocre, paulo longius quam latius, 

 modice constrictum ; semicellulae bilobae apicibus concavis, lobis dichotomis brachiis 

 duobus divergentibus glabris et attenuatis, apicibus profunde bifurcatis ; a vertice 

 visse fusiformes, i)olis productis ; a latere visae ovato-lanceolatae ; membrana glabra 

 tenuissima. 



Long. s. proc. 15 /x, c. proc. 42 fx ; lat. s. proc. circ. 12 ^, c. pi-oc. 42 ^t ; lat. isthm. 6*5 ^i ; 

 crass. 7 tx. 



This genus includes but two species, the above and D. hibrackiatum, nobis 

 [=Sfa.upastrMn bibmchiatum, Keinsch, Contrib. Alg. et Pung. t. xvi. fig. 2], with its 

 var. ci/matium, West & G. S. West, in Trans. Linn. Soc. ser. IL, Bot. v. (1895) p.' 74, pi. 8. 

 tig. 28. 



About half the Desmids mentioned in this paper are hitherto unrecorded for North 

 America, many of them being new species; of those that remain, either unrecorded 

 varieties of them are noticed, or attention is called to points previously unobserved 

 concerning them. 



