PRGTOCOCCACE.E. 41 



not even yet closely connected together, but exhibit spaces between 

 them, so that in this stage it resembles P. pertusum. Not until the 

 second day do the cells become closely applied together ; the horns ac- 

 quire their proper shape and length at the same time. All the figures 

 are magnified 400 diameters, 



The number of cells which enter into the composition of a single disc 

 vary in the same species, so that it cannot be accepted as a character. 

 The arrangement and limit of species adopted are those proposed by 

 Brauii (" Algarum unicellarum," 1855). 



Plate XVI. figs. 1 to 7. Development of Pediastrum, after Braun; 8 

 a, b, zoogonidia. 



SECTION 3. DIACTINIUM. Braun. 

 Cells of periphery emarginate or bilobate, lobes entire. 



Pediastrum selenaea. Kutz. Rabin. Alg. in. 73. 



Coenobium orbicular, entire, formed of 8-16 (rarely 31) cells. 

 Cells of periphery narrow, lunate, acutely lobed ; cells of disc 

 slightly excised, central one five-angled ; substance firm, rather 

 thick. 



SIZE.* Coenobium *028-'085 mm. diam. 



Braun Unicell. Alg. p. 83. Ralfs Desm.t. 21, f. 5. 



Pediastrum Napoleonis, Ralfs Ann. N. Hist. xiv. (1844) t. 

 12, f. 6. 



Pediastrum elegans, Hassall Alg. t. 86, f. 19. 



Pediastrum lunar e, Hassall Alg. t. 92, f. 3. 



In bogs, moor pools, &c. 



Plate XVI. jig. 9. a, 8-celled coenobium ; b, 16-celled ccenobium X 

 400 diameters ; c, marginal cell. 



Pediastrum angulosum. Ehr. Ralh. Alg. in. 73. 



Ccenobium orbicular, oblong, or subreniform, continuous, 

 composed of 8-16-32-64 cells. Cells all even, angular, those 

 of the periphery truncate at the base and dilated upwards, more 

 or less deeply notched in the middle, the lobes obliquely trun- 

 cate, outer angle very shortly apiculate, inner one ending in a 

 shorthorn. Central cells 5-6 angled, slightly repand in front, 

 marked with a small transverse oblong pallid spot. 



SIZE. Ccenobium '12 mm. diam.; cells '019 diam. 



Hass. Alg. t. 86, f. 14. Ralfs Desm. t. 31, f. 11 a b. 

 Braun Unic. Alg. p. 84. 



Pediastrum Boryanum, Ralfs Ann. Nat. Hist. xiv. (1844) t. 

 12, f. 7, upper. 



Pediastrum excavatum, Hassall Alg. t. 92, f. 6. 



In bogs. 



Plate XVI. fig. 10. a, marginal cells ; 5, 16-celled coenobium ; c, 8-celled 

 ccenobium X 400 diameters. 



* The size must depend on the number of cells of which the ccenobium. 

 is composed ; hence throughout this genus the dimensions given must only 

 be accepted as approximate. 



