118 FRESH-WATER ALG^ OF THE UNITED STATES. 



T. leTis, (KuTz.) Ralfs. 



T. Bribissonii formis similis sed parvior, 3-4 plo longior quam latus ; cytiodermate plerumque 



levissimo, interdum indistinctissime punctato. 

 Dmm.—TjV/ = -00066". 



Syn. — T. levis, Kutzinq. Ralfs. Rabenhoest, Flora Europ. Algarum, Sect. III. p. 140. 

 ffab In aquis quietis, prope Philadelphia. 



Similar in form to T. Br^bissonii, but smaller, 3-4 times longer than broad ; cytioderm mostly 

 very smooth, sometimes indistinctly punctate. 



BemarJcs.—'Prof. Rabenhorst states that the cytioderm of this species is very- 

 smooth, and Mr. Ralfs says that he has failed to detect any punctations, but also 

 states that " Mr. Jenner and Mr. Ross assure me that they (punctae) are scattered 

 as in T. granulatus." I have no doubt of their existence in certain individuals, 

 whilst in other cases they appear to be absent. 



Genus PLEUROT^NIUM, N^geli (1849). 



Cellulse singulse in aqua natantes, rectse vel subrectae, cylindricae vel fusiformes, valde elongatas, 

 utroque polo rotundataa vel truncatae, medio leviter constrictae, ex transverso circulares. Cytio- 

 plasma chlorophyllaceum in laminis longitudinalibus pluribus dispositum, et sub utroque polo locello 

 rotundato corpusculis se vivide moventibus impleto instructum. 



Cells single, swimming in water, straight or nearly so, cylindrical or fusiform, very much elongate, 

 rounded or truncate at each end, in the end view with a circular outline. Chlorophyllous protoplasm 

 arranged in longitudinal laminae and furnished at each end with a round vacuole containing actively 

 moving corpuscles. 



Bemarhs. — This genus appears to include the main portion of the species, which 

 have been described under the name of Docidium ; the remainder being represen- 

 tatives of a number of genera. I have not had access to the original description 

 of Docidium, and do not know in what year it was published ; but, according to 

 De Bary, Docidium is much the older name ("Ueber de Conjugat.," p. 75). M. 

 De Bary states, however, that he prefers the name of Naegeli, because that autho- 

 rity first defined the genus and his name expresses very clearly the character 

 of it, as well as from the circumstance that the name Docidium having been 

 made to cover a heterogeneous mass of species, its retention might cause confu- 

 sion. I confess to thinking that this action of De Bary is not in accordance with 

 the recognized laws of priority, but, in the absence of the original description, 

 have thought best to follow it. 



P. trabecula, (Ehrb.) N^qeli. ' 



P. saepe valde elongatum, octies vicies-longius quam latum, cylindraceum, utroque fine laevissime 



attenuatum aut incrassatum, juxta medium constrictum saepius bigibbum (quasi biundatnm), 



apicibus late truncatum ; cytiodermate tenui laevi, achroo. (R.) 

 I)iam.—jy = .Q0\2,". 



8yn.—" Docidium Ehrenbergii. Ralps." Bailey, Microscopical Observations. Smithsonian 

 Contributions. 



Pleurotsenium trabecula, (Ehr.) NiBOELi. Rabenhoest, Flora Europ. Algarum, Sect. 

 III. p. 141. 



Bab.— Sonth Carolina, Georgia, Florida ; Bailey. Pennsylvania ; Wood, 



