158 Rhodora [SEPTEMBER 
of St. Lawrence. June 16, 1907, the writer found on the bay shore of 
Eastham, Massachusetts, growing on a spider crab, Libinia canali- 
culata Say, what appears to be S. fusca. S. cirrhosa is a variable 
species, and as regards most characters it is hard to draw a line between 
it and S. fusca; the most conspicuous character is found in the pro- 
pagula; in S. eirrhosa the three rays are sharply contracted at the 
base; in S. fusca there is no such contraction, the rays being cylindri- 
cal or tapering slightly from base to summit. 
PrErRocELIS Мтррехровғеп (Rupr.) Kjellman, Algae of the Arctic 
Sea, р. 140, 1883. "Тһе first notice of the occurrence of Petrocelis 
on the New England coast is by Farlow * under the name of P. cruenta 
J. Ag.; this name has since been retained in all references to the plant 
of this region. А comparison with a specimen from Cherbourg, 
France, collected and determined by Le Jolis, shows so much differ- 
ence that the two cannot be considered as belonging to the same 
species. The frond in Petrocelis forms an incrustation on rocks at 
or near low water mark on exposed shores; it is thin but not as thin as 
the frond of the common Hildenbrandtia prototypus Nardo; the latter 
is more or less translucent, the color being a lighter or darker red 
according to the color of the substratum. ‘The Petrocelis frond is 
quite opaque and usually of a duller red. It consists entirely of vertical 
filaments, in which two parts can be distinguished; a lower layer in 
which the filaments are firmly united laterally, and an upper layer in 
which they are in contact, but do not adhere. On the top is a rela- 
tively firm cuticle. When the cover glass is pressed down on a bit of 
the plant under the microscope, it is common for the filaments of the 
upper part to be held by the basal part below and by the cuticle above, 
while separating easily between. In the European plant the basal 
part is only a small portion of the whole thickness, while in the New 
England plant it is seldom of less thickness in a mature plant than the 
upper part, and often it is two thirds of the whole thickness. In the 
European plant the cells of the free filaments are moniliform, 1-2 
diam. long, about 8 diam. at the base, diminishing to 4 y: at the top. 
In the American plant the filaments are cylindrical or only slightly 
tapering, 3-4 и diam, cells up to З diam. long. ‘These characters 
indicate that our plant is P. Middendorffii, a plant probably of circum- 
polar distribution, as it occurs in Norway, and in the Pacific from 
Alaska to California. 
1 Farlow, N. E. Marine Algae, p. 115, 1881. 
