228 
Rhodora [DECEMBER] 
plast with radiating plates, entire at the outer edges, each chloroplast 
with one or more pyrenoids, when several, in an axile series; cell wall 
usually with pores. 
II 
KEY ro THE New ENGLAND SPECIES or PENTIUM. 
Cells unconstricted, smooth. 
1. Cells large, breadth 20-50 yx í ; Tet P. Libellula. 
2. Cells small, breadth 10-15 p ; à i : P. Navicula.. 
Cells usually constricted in the middle . 
1. Cell wall granulate. 
A. Cell wall granulate over its entire surface. 
a. Cells large, usually more than 100 (long P. margaritaceum. 
b. Cells minute, usually less than 30 » long . P. cuticulare. 
B. Granules restricted to the ends of the cell à : P. Clevei. 
2. Cell wall striated. 
A. Cells large, striations distinct, spirally arranged P. spirostriolatum. 
B. Cells small, striations longitudinal and indistinct. 
a. Breadth usually more than 20 p . ; P. polymorphum. 
b. Breadth usually less than 20 y ; . P. phymatosporum. 
3. Cell wall smooth or punctate. 
A. Cells 2-4 times as long as broad. 
a. Cell wall punctate : , i ; ; P. curtum.. 
b. Cell wall smooth í : i ^ j P. cruciferum. 
B. Cell elongate, many times as long as broad A . P. minutum. 
4. Cell with annulations . ; ; ; : : P. annulare. 
Pentium LiBELLULA (Focke) Nordst. 
Closterium Libellula Focke, Physiologische Studien, 1847, p. 58, 
pl. 3, fig. 29. 
Penium Libellula Nordst., Videnskab. Meddel. f. d. naturh. Foren. 
E. 
Jóbenhavn, 1888. p. 184; W. & G. S. West, Brit. Desm., vol. 1, 
1904, p. 73, pl. 7, figs. 6, 7. 
Penium closterioides Ralfs, Brit. Desm., 1848, p. 152, pl. 34, fig. 4; 
Wood, Smithson. Contrib. to Knowl. No. 241, 1874, p. 109; Wolle, 
Desm. U. S., 1884, p. 35, pl. 5, fig. 18. 
Cells large, 43-63 times as long as wide, fusiform, no constriction, 
ends subtruncate, polar vacuoles with several moving granules. 
Length 186-233 i; maximum breadth 38-50 p. 
N 
. H.: Intervale, scarce! Pudding Pond, North Conway, common! 
