THE MARINE ALG.E OF NEW ENGLAND. 115 



P. Dubyi, Crouan; Phyc. Brit., PI. 71; Flomle du Finistere, PI. 19, 

 Fig. 130; Proc. Am. Acad. Arts & Sciences, 1ST 7, p. 239. 



Fronds dark purple, thin, completely adherent to the substratum, 

 somewhat calcareous beneath ; cystocarpic spores few in number (1-0), 

 arranged in one or two rows. 



On shells and stones at low-water mark and in deep water. 



Eastport, Maine ; Magnolia, Mass. ; Europe ; California. 



As yet only found in a sterile condition, apparently not common. The species 

 might possibly be mistaken for Pctrocclis cruenta at first sight. It is, however, more 

 decidedly reddish and thicker. Under the microscope the structure of the frond is 

 seen to be parenchymatous throughout, while in Petrocelis the vertical filaments are 

 nearly free from one another. P. imbricata, Kiitz., Tab. Phyc, PL 90, from Newfound- 

 land, is a doubtful species, which is not likely to be recognized by future botauists. 



PETROCELIS, J. Ag. 

 (From nerpog; a stone, and ktj?u-, a stain.) 

 Fronds gelatino-coriaceous, horizontally expanded, indefinite in out- 

 line, adhering closely to the substratum, vertical filaments united below, 

 but above rather loosely held together by a gelatinous substance; 

 antheridia and cystocarps unknown ; tetraspores spherical, cruciate, 

 formed directly from some of the cells of the vertical filaments. 



A genus represented by a single species, which is widely diffused in the North At- 

 lantic. At once recognized by the peculiar position of the cruciate tetraspores, which 

 are in the continuity of the vertical filaments. There is usually only a single tetra- 

 spore in each filament, but Ruprecht, in Phycologia Ochotensis, figures a form in which 

 several contiguous cells are transformed into tetraspores. 



P. cruenta, J. Ag. (Cruoria pellita, Harv., in Phyc. Brit., PI. 117, 

 non C. pellita, Lyngb.) PL 14, fig. 1. 



Covering rocks and stones near low-water mark with a dark purple, 

 velvety stain. 



Common from Nahant northward; Europe. 



The present species often accompanies HildenbrandUa rosea, from which it is dis- 

 tinguished at sight by its darker color and velvety gloss when moist. It is also de- 

 cidedly thicker aud more easily scraped from the rocks. The species is not yet known 

 south of Cape Cod, but may be expected. The fronds of the present species are 

 infested by a green unicellular parasite, which is frequently seen in the shape of ovoid 

 sacks, drawn out at the lower end into a slender stalk amongst the vertical filaments. 

 It is, in all probability, the parasite mentioned by Cohn, in Ueber einige Algeu von 

 Helgoland, as occurring in Cruoria pellita, to which, as far as we know, no name has 

 as yet been given. 



HILDEXBRANDTIA, Xardo. 



(Named in honor of Prof '. Franz Edler Hildenbrandt, of Vienna.) 

 Fronds crustaceous, without calcareous deposit, forming thin, reddish, 

 horizontal expansions of indefinite extent, composed of cuboidal cells 

 arranged in vertical lines and arising from a horizontal basal layer; 



