202 PROCEEDINGS OF SOCIETIES. 



but are not regularly moniliform as they are in L. tuberiformis and L. 

 umbellata. 



At first I was disposed to think this the same as L. umbellata, Ag., 

 but on comparing them together under the microscope the difference in 

 the peripheric ramelli, added to the different habitat, appears to forbid 

 their being united. I have received Z, umbellata from Kiltzing, Zanar- 

 dini, and Lenormand, in all. cases growing on the receptacles of Cystoseira 

 granulata. 



Plate XII. A, Fig. 1, Leathesia crispa growing on Chondrus crispus, 

 the natural size. Fig. 2, part of a filament, with its curled apical ra- 

 melli, magnified. Fig. 3, ramellus and spore, more highly magnified. 



2. Elachista Grevillii (Am.) — Tufts pencilled; filaments somewhat 

 rigid, elongate, slender, cylindrical, slightly narrowed at the base, and 

 scarcely attenuate at the apex; lower articulations shorter than their dia- 

 meter, upper as long as broad ; tubercle minute. — Tab. XII. B. 



On Cladophora rupestris, at Largs (Dr. Greville), July, 1852; and 

 at Corrighills, Arran (Professor Walker Arnott), the same year. 



Similar in many respects to E. fueicola, but smaller, with shorter 

 joints, and arising from a much smaller tubercle. It is remarkable, too, 

 for its habitat, growing on one of the Chlorospermatous Algae, whose 

 fronds it infests often as densely as C. fueicola does those of the Fuci. 

 The habitat of this species seems sufficiently to refute an opinion which 

 has sometimes been maintained, that the Elachistae are merely abnor- 

 mal cellular growths of the plants on which they are found. Such an 

 inference might appear plausible in the case of species growing on the 

 Fuci or the Dictyotece, whose cells are of a similar character ; but the 

 differences between the cells of Cladophora rupestris and those of the 

 parasite are too great to allow of their being confounded. Other cha- 

 racters apart, then, the habitat of the present little Alga induces me to 

 give it a local habitation and a name. 



Plate XII. B, Fig. 1, Elachista Grevillii growing on Clad, rupestris, 

 the natural size. Fig. 2, portion of the tuft magnified. Fig. 3, apex 

 and base of a filament more highly magnified. 



3. Actinococctjs Hennedyi. — Frond effused, indefinite in outline ; 

 filaments simple, or slightly branched, attenuated upwards ; articula- 

 tions once and a half as long as broad ; tetraspores large, cruciate, qua- 

 drate.— PL XIII. A. 



On an old root of Laminaria digilata, at Cunibrae (Mr. lloger Hen- 

 nedy), 1852. 



I should have supposed this to be A. simplicifilum (J. Ag.), bat that 

 the frond or crust is indefinitely extended, and the filaments are fre- 

 quently once or twice branched ; the articulations are nearly similar. 

 The tetraspores in our plant are very large, forming beautiful necklace- 

 like strings of deep-red beads. 



The specific name is given in honour of its discoverer, Mr. Roger 

 Hennedy of Glasgow, a most acute and successful explorer of the minute 

 Algae. 



Plate XIII. A, Fig. 1, magnified view of a portion of the crust of Acti- 



