THE DIATOMACE.E OF PHILADELPHIA AND VICINITY 113 



RHOPALODIA VENTRICOSA (KUETZ.) MUELLER 



Valve gibbous in the middle on the dorsal side, straight on the ventral side, with 

 reflexed apices; costse, 7 in 10 ju; striae, 14-16 in 10 n- L. 40-100 n. 



The median nodule appears as a minute depression in the middle of the dorsal side. 

 The two species usually occur together. 



Epithemia gibba var. ventricosa Kuetz. 



PL 31, Fig. 24. 



SURIRELLOIDE.E 



The Surirelloidese are usually understood to include the genera Surirella, Podocystis, 

 Cymatopleura and Campylodiscus, all of which resemble each other more or less, either 

 in having a keel or markings like the divisions of the keel in Surirella and a median line, or 

 pseudoraphe. The genus Nitzschia also has a keel, but it does not border each side of the 

 valve as in Surirella, being found either near one margin or between it and the centre. 

 Certain of the Surirellae are allied to the group Tryblionella of the Nitzschise, while forms 

 of Stenopterobia are distinguished with difficulty from the group Sigmata. 



The following arrangement, therefore, is intended to include all genera having a keel 

 or something which resembles it. 



Hantzschia. Valve asymmetrical ; keels of the two valves opposite each other. 



Nitzschia. Valve asymmetrical; keels not (usually) opposite each other. 



Surirella. Valve usually symmetrical; a keel on each border. 



Cymatopleura. Valve without an elevated keel, but with markings like those of Suri- 

 rella; undulated in zone view. 



Campylodiscus. Valves saddle-shaped. 



HANTZSCHIA GRUN. (1377) 

 (named after C. A. Hantzsch) 



Valve arcuate, with rostrate ends; keel puncta short, prolonged into costse or extend- 

 ing across the valve; median nodule rudimentary; the keels of the two valves opposite 

 each other. 



Distinguished from Nitzschia chiefly by the position of the keels. According to 

 Mereschkowsky, however, two species of Nitzschia, N. lanceolata and N. spectabilis, show 

 the same peculiarity. 



Chromatophores four, two on each of the zones (Mereschkowsky). 



HANTZSCHIA AMPHIOXYS (EHR.) GRUN. 



Valve slightly arcuate, with rostrate apices; keel puncta, 8 in 10 M; striae transverse, 

 16-18 in 10 M, punctate. L. 60 . 

 Quite variable. 

 Fresh water. 

 PI. 32, Fig. 9. 



