FRESH-WATER ALG^ OF THE UNITED STATES. 107 



Bemarhs. — This, species is probably widely diffused through the temperate por- 

 tions of North America. I have found it abundantly near Philadelphia, as well as 

 among the Alleghanies, and have received specimens from Dr. Lewis, collected in 

 Saco Lake, Northern New York ; Prof. Bailey also notes it as occurring in Georgia. 

 There is one form of it which resembles somewhat in outline the modern coffin, 

 one end being much broader and much more rapidly narrowed than the other. 

 There is no distinct vacuole at the end, at least in any specimen I remember to 

 have seen, although frequently large numbers of moving granules can be detected 

 in that portion of the frond. 



Fig. 6, pi. 20, represents the outline of a frond of this species. 



P. lamellosum, Bs£b. 



P. cellulis oblongo- vel fusiformi-cylindricis, diametro 5-6 plo longioribus, medio ssepe leviter 

 constrictis, utroque polo magis atteuuatis, obtuso rotundatis. (11.) 



i?mm.— 0.0023"— 0.0029"'. (R.) 



Syn. — P. lamellosum, Bbeb. Rabenhobst, Flora Europ. Algarum, Sect. III. p. 118. 



Hab. — Rhode Island. (Olney) Thwaites. 



Cells oblong or fusiform cylindrical, 5-6 times longer than broad, often slightly constricted in 

 the centre, more attenuate at the ends, obtusely rounded. 



Bemarhs. — I have never recognized this species, but it is one of those sent over 

 by Mr. Olney, and identified by Prof. Thwaites. 



h. Lamina chloro'phyllacea integerrima. 

 b. Chlorophyl lamina entire. 



* Gellulae in medio plus minus constrictae. 



* Cells more or less constricted in the middle. 



P. margaritacenm, Ehbb. 



P. elongato cylindricum, diametro 8-9 plo longius, medio plerumque leviter constrictum, utroque 

 polo rotundato-truncatura ; cellulse membrana nodulis seriatis quasi margaritacea ; locellus 

 in medio (circiter) utriusque cruris corpusculis mobilibus in more Closteriorum repletus. (R.) 



Diom— 0.00098"— 0.0011". (R.) 



^a6.— Rhode Island. (Olney) Thwaites ; Bailey. Florida. Bailey. 



Elongate cylindrical, 8-9 times longer than broad, in the centre generally slightly constricted, 



at each end roundly truncate ; membrane of the cells somewhat pearly with seriate granules ; 



vacuole about in the centre of each crus, filled with moving granules, as in closterium. 



BemarJcs.—l have not seen this desmid, hut it is in Prof. Bailey's list; it was 

 also among those sent by Mr. Olney to Prof. Thwaites. 



P. minntum, Cleve. 



P. cylindricum, gracile, diametro 5-1 plo longius, Iseve, ad polos obtusissimos (latissime rotun- 

 datos) parum attenuatum, medio leviter constrictum. (R.) Species mihi ignota. 



Z)iam.— 0.00044"— 0.00063". (R.) 



Syn. Docidium minutum, Ralf's British Desmid. 



P. minutum, Cleve. Rabenhorst, Flora Europ. Algarum, Sect. III. p. 122. 



