THE MAEINE ALGJE OF NEW ENGLAND. 39 



preceding species in having filaments .008 mm to .012 mm in diameter, was recognized by 

 Dr. Bornet in company with B. plicata in a specimen from Cohasset Narrows. As we 

 have not been able to recognize the species in any of our own specimens from the 

 same locality, the presumption is that it is not very common. 



ISACTIS, Thuret. 



(From taog, equal, and unrig, a ray.) 



Frond plane, composed of parallel filaments, field togetfier by a tougfi, 

 gelatinous intercellular substance, ending in a fiyaline fiair, fietero- 

 cysts basal, ramifications few. Spores unknown. 



This genus differs from Bivularia only in that the filaments are parallel to one 

 another so as to form a flat frond, whereas in Bivularia they radiate from a central 

 point and form more or less spherical fronds. It might with propriety be considered 

 a subgenus under Bivularia. 



I. plana, Tfiuret, 1. c. (Dasyactis, Kfitz. — Physactis atropurpurea, 

 obducens, Kfitz.) PI. I, Fig. 2. 



Frond flat, tfiin, dense, dark green, outline irregular, filaments 

 .0076-95 mm in diameter, .12-.15 mm liigfi sfieaths often torn and striate. 



Wfiole New England coast ; Europe. 



Very common on rocks, Fucus, Punctaria, and other algse, forming dark-green spots, 

 scarcely raised above the substance on which it is growing. 



HOEMACTIS, Tfiuret. 



(From bpfiog, a necklace, and unrig, a ray.) 



Frond gelatinous, globose, at first solid, tfien fiollow and plicate, 

 lieterocyst intercalary, filaments simple at tfie surface of frond, bifur- 

 cating below. Spores unknown. 



H. Quoyi, (Ag.) Bornet, in litt. (Bivularia nitida, Farlow, List of 

 Marine Algae, 1876. PL II, Fig. 1. 



Fronds gregarious, dark green, plicato-rugose, from a quarter of an 

 incfi to two to tfiree incfies in diameter, filaments .0028-55 mm in diam- 

 eter, tortuous, cells of external part of tfie frond tfiick and discoidal, 

 becoming more oval in tfie interior of tfie frond fieterocysts numer- 

 ous, scattered, about .003S mm x .0058 mm . 



Exs. — Alg. Am. Bor., Farlow, Anderson & Eaton, No. 45. 



Wood's Holl, Mass., W. G.F.; Falmoutfi, Mass., Mr. F. S. Collins; 

 Marianne Islands. 



This interesting species, although it has as yet only been found at Wood's Holl and 

 the adjoining coast, will probably also be found at other localities on Long Island 

 Sound. It grows in considerable quantities upon species of Fucus, at half tide, on the 

 inner side of Parker's Point, Wood's Holl, and we have also found it washed ashore 

 on the beaches of Buzzard's Bay, in the same township. It makes its appearance in 

 June, and disappears in the month of September, being in perfection in the month of 

 July. The fronds sometimes acquire a large size, two or three inches in diameter, 

 but usually they are much smaller and densely aggregated, almost covering the Fucus 



