OSCILLATORIACEiE. 
99 
I. PETALONEMA, Berk 
Filaments stratified, decumbent, free, simple or branched. Tube or sheath very wide, 
flattened, longitudinally and transversely striate, and crenulate at the edge ; endochrome 
olivaceous, annulated, here and there interrupted by a heterocyst. Branches issuing in 
pairs, formed by the division and protrusion of the endochrome of the original filament. 
A very distinct and easily recognized genus established by Mr. Berkeley in 1833, 
under the name here adopted ; a name changed by Professor Kiitzing in 1845 to 
A rthrosiphon , for what reason I am not aware. The Alga on which it is founded was 
discovered many years previously, in the West of Scotland, by the late Captain Car- 
michael, and was first figured and described by Dr. Greville as an Oscillatoria. It has 
more recently been found in several parts of Europe, and we have now to record its 
occurrence in the New Continent. There are few more beautiful objects among the 
fresh water Algae, and unlike many of its kindred the fronds perfectly recover their 
form when moistened after having been dried. When placed under the microscope the 
filaments present the appearance of a cylindrical central column, containing annulated, 
olive-coloured endochrome, and a wide winglike border at each side of the column. This 
border or sheath is obliquely striate, the striae running in an arch from the margin 
toward the centre, where they become parallel, and are then continued longitudinally 
downwards along the medullary column, till lost in the density. The margin of the 
wing is closely crenulate, and in age transversely striate at the crenatures as if jointed. 
Such is the apparent structure : the real structure seems to be, that an annulated 
central filament is enclosed within a number of compressed, trumpet-mouthed gelatino- 
membranaceous tubular sheaths, one arising within the other, and successively developed 
as the growth proceeds. These sheaths, thus concentrically arranged, are indicated by 
the longitudinal arching strias ; and the mouths of the younger sheaths, projecting 
slightly beyond those of the older, form the crenatures of the margin. 
Petalonema alatum , Berk. Gl. Br. Alg. p. 23, t. 7, Jig. 2. Harv. Man. Ed. 1 , 
p. 168. Hass. Fr. Wat. Alg. p. 237. t. 68./. 6. Arthrosiphon Grevillii , Kutz. Phyc. 
Germ. p. 177. Sp. Alg. p. 311. Oscillatoria alata , Carm. Grev. Sc. Crypt. FI. t. 
222. Harv. in Hook Br. FI. 2. p. 378. (Tab. XLYIII. A.) 
Hab. On dripping rocks, under Biddle Stairs, Niagara Falls, abundantly, W. H. H. 
(1849). (v. v.) 
This forms strata of a dark chestnut -brown colour, and of indefinite extent, on the 
surface of rocks or soil in places exposed to the constant drip of water. The jilaments 
are decumbent, lying without order in the gelatinous matrix in which they are deve- 
loped, and which forms the groundwork of the stratum. They appear to be unattached 
to the soil, and each filament may be about half an inch in length ; but they are com- 
monly found broken off at the inferior end, or the lower portion decays while the upper 
continues to grow. They are slightly curved, in serpentlike fashion, never quite 
