174 FRESH-WATER ALG^E OF THE UNITED STATES. 



. 

 SCENEDESMUS ROTUNDATUS, Wood. 



Cells globose or subglobose, armed with three to five very 

 long, slender, acute, straight spines, single or in pairs, or 

 three to four closely conjoined in a two-fold rank. 



Diameter, 4-8 //. 

 Plate CLVI, fig. 27. 



Dr. Wood's remark to this form, "It seems scarcely 

 correct to place this plant in the germs Scenedesmus, but I do 

 not know any other genus to which it is more closely allied." 

 Wood's Contribution, p. 91. 



This form is not infrequently intermingled with other 

 species of Scenedesmus; it certainly appears related, but 

 whether a distinct species, or merely connected in some way 

 in a process of development, has not been made clear. 



II. PSEUDO-COENOBIA.- -The single cells united into a coeno- 

 biuni-like family, but separated from a true coenobium by the 

 cells not all belonging to the same generation. 



Genus 46, SCIADIUM, A.Br. 



Family is composed of a number of cylindrical cells, each 

 contracted at the base into a short slender stem by which they 

 are united, causing the long cells to spread above. Gonidia six 

 to eight, resulting from the division of the cell contents ; at 

 length protruding from the ruptured apex, retained at the 

 mouth and extending in the form of an umbel. This process is 

 repeated to a third, or sometimes a fourth generation. Ultimate 

 cells produce free biciliate zoogonidia. 



SCIADIUM ARBUSCULA, A. Br. 



Umbellate, cells straight, rarely falcate, obtuse at the 

 apex ; contracted at the base into a short stem whose length 

 is about the same as the diameter of the cell. 



Diameter of cells 4 yw, with exceptions 3-5 yw. 



Syn. Ophiocytium arbuscula, (Naeg.) Bab. 



Rather rare, but had single specimens from three or four 

 States. In small pools. 



Plate CLVII, figs. 1, 2, young cells ; fig. 3, commencement 

 of the first generation of daughter cells ; fig. 4, farther pro- 

 gress of the progeny ; fig. 5, a second generation being 

 evolved from the first ; fig. 6, a third generation developing 

 from the last. 



