42 THE DIATOMACE^E OF PHILADELPHIA AND VICINITY 



DIATOMA VULGARE BORY. 



Valve elliptical-lanceolate, with apices sometimes rostrate or capitate; pseudoraphe 

 narrow; costae, 5 in 10 M- 



Common everywhere in pure fresh water and extremely variable. 



PL 10, Figs. 9 and 10. 



Var. elongatum (Ag.) = var. ehrenbergii (Kuetz.) elliptical-lanceolate, constricted 

 near the apex. 



Var. grande (Wm. Sm.) Grun. linear, elongated, constricted near the apices. 



PL 10, Fig. 4. 



Both of these varieties, with numerous intermediate forms, are abundant near New- 

 town Square. Varieties of Grunow, known as breve, ovate-lanceolate; productum, ovate- 

 lanceolate with produced apices; capitulatum, lanceolate with capitate extremities, are 

 mingled together in the same gathering. 



DIATOMA ANCEPS (EHR.) KIRCHN. 



Valve linear with rostrate apices; costae robust; striae delicate, 20 in 10 M- Zone view 

 quadrangular. 



Common in fresh water. 



PL 10, Figs. 5 and 6. Fig. 11, PL 40, shows frustules containing the nuclei and chroma- 

 tophores. 



DIATOMA HIEMALE (LYNG.) HEIB. 



Valve ovate-lanceolate to lanceolate; apices obtuse, not produced. Costae not num- 

 erous, robust; striae moniliform. Zone view quadrate, the costae as septa deeply dividing 

 the valve into convex elevations. 



Common in springs. 



PL 10, Figs. 7 and 8. 



In all species of Diatoma a punctum, or pore, is observed, usually at alternate ends of 

 the two valves, by means of which a communication exists between adjoining frustules 

 and causes them to adhere in zig-zag chains when partially separated. 



PLAGIOGRAMMA GREV. (1859) 

 (plagios, on the side, and gramma, a letter) 



Frustules quadrangular, adnate in fasciae, or free. Valve linear, elliptical, or elliptical- 

 lanceolate, divided by two or more median and two terminal costae or with a central and 

 two terminal hyaline spaces. 



Valve with two median and two terminal costae : 



Linear, pseudoraphe distinct pygmeeum 



Linear, with striae at the ends wallichianum 



Ovate-lanceolate obesum 



Valve without costae but with central and terminal nodules : 



pseudoraphe absent tessellatum 



