THE DIATOMACE.E OF PHILADELPHIA AND VICINITY 19 



PYXIDICULA EHB. (1833) 

 (dim. of pyxis, a box) 



Frustules globular, solitary or in short fasciae. Valve more or less hemispherical, 

 areolate, destitute of spines. 



PYXIDICULA CRUCIATA EHR. 



Valve hemispherical, with large, hexagonal cells. An inner stratum is finely punctate. 



Blue clay. Walnut St. Bridge. Rare. 



PL 38, Fig. 8. 



This form is not usually described as having punctate areolse, but it does not apparently 

 differ from other forms of Pyxidicula of Ehrenberg as described by Kuetzing (Species Alga- 

 rum, pp. 21-23), including P. areolata. In fact, it differs from Stephanopyxis, which is also 

 sometimes punctate, only in the absence of spines. In fossil deposits the absence of an 

 easily detached stratum is not significant. The difference, except in size, between it and 

 P. mediterranea Grun. (V. H. S., PI. 95, Figs. 15 and 16), I am unable to determine. 



Although many species of Meloseira are fresh-water, the habitat of the group Melo- 

 seirinse is, in general, marine. It more nearly coincides in structure and development with 

 other algae not diatomaceous, the siliceous envelope constituting its most distinctive feat- 

 ure. As we proceed in the classification, the structure both of the frustule and contents 

 becomes more complicated. 



(6) COSCINODISCIN^E 



1. Cyclotella. Valve with two concentric divisions of different structure, one a wide 

 border and the other a central surface. 



2. Coscinodiscus. Valve areolate or punctate, with a narrow border of the same 

 structure. 



CYCLOTELLA KUETZ. (1833) 

 (cycles, a circle) 



Frustules single or geminate, cylindrical, short, in zone view rectangular or with undu- 

 lating sides. Valve usually with smooth or punctate striae, centre sometimes bullose, smooth, 

 or with granules scattered or radiating. 



Chromatophores numerous along the valves (Pfitzer). 



CYCLOTELLA STRIATA (KUETZ.) GRUN. 



Valve 30-80 ju in diam., with coarse striae, 7-12 in 10 p, centre coarsely punctate and 

 bullose. 



Coscinodiscus striatus Kuetz. 

 Cyclotella dallasiana Wm. Sm. 

 Common in the blue clay. 

 PL 2, Fig. 9. 



CYCLOTELLA MENEGHINIANA KUETZ. 



Frustule in zone view rectangular, undulated; valve, 10-20 n in diam., marginal striae 

 robust and transversely punctate, centre radiately punctate. 

 Cyclotella kuetzingiana Wm. Sm. (not Thwaites). 

 Crum Creek. 

 PL 2, Fig. 8. 



