THE MARINE ALGM OF NEW ENGLAND. 187 



Fronds small, tufted, composed of a dense basal portion and an 



outer portion composed of free filaments 28 



Fronds tubular, unbranclied . . 29 



Fronds filamentous . 31 



Fronds membranaceous, expanded 41 



27. Fronds densely parenchymatous throughout, fruit in external 



spots . Balfsia. 



Fronds minute, thin, formed of a basal horizontal layer of cells and 

 short vertical filaments, between which the sporangia are borne. 



Myrionema. 



28. Free filaments all alike s Myriactis.* 



Free filaments of two kinds, one short and the other exserted. 



Eloehistea. 



29. Fronds simple, hollow throughout, substance thin 30 



Fronds simple, cylindrical, somewhat cartilaginous, with numerous 



diaphragms \ Chorda. 



Fronds brandling, substance thin, sporangia large, arranged in 

 transverse lines ,...*. Striaria. 



30. Sporangia densely covering the surface , . . . Scytosiphon. 



Sporangia external in scattered spots . , Asperococcus. 



31. Fronds capillary, branching, formed of single rows of cells (mono- 



siphonous) ... , . . . Ectocarpus. 



Fronds cylindrical, solid or occasionally becoming partially hollow 

 with age 32 



32. Fronds slimy, composed of an axial layer of elongated filaments 



and a distinct cortical layer of short, horizontal filaments 33 



Fronds composed of elongated internal cells, which become smaller 



and polygonal at the surface 35 



Fronds, at least in the younger portions, formed of cells of nearly 



uniform length, arranged in transverse bands, without any proper 



cortical layer 38 



33. Fronds tough and dense Chordaria. 



Fronds soft and flaccid 34 



34. Outer cells of cortex producing plurilocular sporangia . . Castagnea. 

 Outer cells of cortex not producing plurilocular sporangia. Mesogloia. 



35. Fronds traversed by a central filament formed of large cylindrical 



cells placed end to end 36 



Fronds destitute of distinct axile filament 37 



36. Sporangia in branching, monosiphonous filaments, which form tufted 



whorls on the branches . Arthrocladia. 



Sporangia inconspicuous, formed from the cortical cells. Desmarestia. 



37. Sporangia globose, prominent in the cortical layer Dictyosiphon. 



* When reference is made in Myriactis and the following genera of Phceosporecs to 

 free external filaments, it should he understood that only filaments whose cells contain 

 coloring matter are meant, and that no account is to he taken of the numerous hyaline 

 hairs with which most of the species of Phcoosjiorcce are covered at certain seasons. 



