THE DIATOMACE^E OF PHILADELPHIA AND VICINITY 67 



AMBLYAMPHORA 

 AMPHORA OBTUSA GREG. 



Frustule rectangular. Valve linear, obliquely rounded at the ends, with arcuate dor- 

 sal, and straight ventral, margin; median line biarcuate; striae, 18-20 in 10 /*. 

 Along the coast. Common. 

 PI. 15, Fig. 4. 



PSAMMAMPHORA 

 AMPHORA ARENARIA DONK. 



Frustule hyaline, rectangular, slightly tumid in the middle, with rounded angles. 

 Valve linear with broad ventral side and straight or sinuate ventral margin. Striae, 24-27 

 in 10 M (Cleve). 



Common along the coast. 



PL 15, Fig. 17. 



The distinction between A. obtusa and A. arenaria is not always evident if the valves 

 alone are seen. The former has a complex zone, the latter a simple zone, and the valve 

 has finer striae. Cleve's descriptions and references in regard to these two forms do not 

 agree with the descriptions and figures of H. L. Smith, or with the figures of Schmidt. The 

 valves of most Amphorae are capable of assuming various outlines according to their 

 position. 



AMPHORA OCELLATA VAR. CINGULATA CLEVE 



Frustule rectangular. Valve linear, with dorsal margin arcuate and the ventral margin 

 straight. Central nodule with a stauros on the dorsal side. 

 Squan River, N. J. 

 PI. 15, Figs. 14 and 15. 



CYMBAMPHORA 

 AMPHORA ANGUSTA VAR. EULENSTEINII GRUN. 



Valve lanceolate, acute at the ends. Median line straight, approximate to the margin. 

 Axial area widened on the dorsal side, indistinct on the ventral; striae punctate. 



A. eulensteinii A. S. 



Common along the coast. 



PI. 15, Fig. 16. 



On PI. 40, Figs. 21, 22, and 23, I have attempted, imitating H. L. Smith's figures 

 (Lens, I.e.), to illustrate the difference in the transverse sections of Navicula, Cymbella 

 and Amphora. 



Fig. 21 represents the transverse section of a convex Navicula, in which the valves 

 ecg and fdh are parallel, and the median nodules c and d are central. 



Fig. 22 is a transverse section of Cymbella in which the valves are nearly parallel and 

 the median nodules are excentric. The girdles on one side, ea and af, are narrower than 

 gb and bh on the other side. 



Fig. 23 is a transverse section of an Amphora in which the valves appear in zone view 

 with the median nodules of both valves on the same side. The girdles on the ventral 

 side, ea and af, are narrower than gb and bh on the dorsal side. The girdles on the 

 dorsal side are seldom as broad as gb and bh, the valve extending over a great part 

 of the dorsal side to g' and h'. 



