FRESHWATER ALGA. 4] 
following respects: the very regular oval or circular shape of the thallus, the invariably 
loose arrangement of the trichomes, the narrow sheaths, the rather wider trichomes 
and in the spores. Unfortunately the authors give no details as to shape or size of 
the heterocysts. 
N. longstaffi appears more nearly related to N. cuticulare (Bréb.) Born. et Flah., 
but there are striking differences, viz., the non-confluent thalli and the heterocysts, 
which are nearly double the size of the vegetative cells. 
Bornet and Flahault’s key to the species of this section may therefore be slightly 
modified as follows :--- 
Sect. 1.—Plante aquaticee, maculiformes, adnate vel postea libere natantes, ambitu 
crescentes ; fila laxe vel arcte contorta . Seo noe costeonse nooo  KCLUAU CLEAN, 
(a) Trichomata 3-4 » crassa, dense slice to- soranciehans 3 Spore ahem 
Heterocystz eadem forma et magnitudine ut articuli vegetativi vel 
paulo crassiores ; sporee spheericze 8-10 p crassee eek eee WV. cuticulare. 
Heterocystze diametro articuli ce ke ca. duplo latiores ; spore 
spheerice, 5 pw crassie.. 500 SEncieeisesesitcesiscciceeiccwenerees 
(b) Trichomata 2°5-4 p crassa, Ibsen spore » Gunes vel depresso- 
globose, in longas catenas seriate. 
Trichomata 8°5-4 1 crassa ; sporee spheericee seepius 6. crassve, in 
longas catenas seriatee ; heterocyste ?............ c weve WV. maculiforie. 
Trichomata 2°5-3°5 pw crassa; spore ies Picnee sal rarius 
globose, in catenis dispositis ; heterocyste diametro articuli 
vegetativi ca. duplo latiores ........1.s1s0e cen eee corer cee voreseveecev one LV. isciforme, sp. n. 
NV. longstaffi, sp. n. 
60. Nostoc HEDERULA. 
Nostoc Hederule Meneghini; Bornet et Flah., op. cit., vii. (1888), pp. 189-190 (N. punetiforme (Kiitz.) ; 
Hariot, Journ. de Bot., v. (1891), p. 29). 
[ab.—Gap pond, Winter Harbour (on Phormidium). 
Minute colonies of very diverse shapes with very densely crowded filaments of 
more or less spherical cells. 
61. Nostoc FUSCESCENS sp. n. 
(Pl. IIL, figs. 138-144.) 
Thallo ab initio globoso, primo adnato(?), postea libere natante, flavo-fuscescenti, 
fusco vel nigro-fuscescenti, zetate provecta plus minusve cavo et interdum lacerato, 
diametro usque ad 10 mm., peridermate tenui levi mucoso facile lacerato circumdato ; 
trichomatibus laxe intricatis, seepe valde contortis, interdum parallelis ; vaginis in thallis 
parvis et in partibus exterioribus thallorum seniorum distinctis, lamellatis, pleramque 
in duas regiones disjunctis, regione exteriore diffluenti hyalina vel dilute flavo- 
fuscescenti plus minusve inter cellulas constricta, regione interiore densiore distincte 
delimitata fusca vel nigro-fuscescenti et inter cellulas valde constricta (seepe simili 
septationis), in partibus interioribus thallorum  seniorum  siepe dilute _ flavo- 
M 2 
