THE DIATOMACE^E OF PHILADELPHIA AND VICINITY 47 



DIMEROGRAMMA MINUS (GREG.) RALFS 



Valve rhombic-lanceolate; striae punctate, radiate; pseudoraphe lanceolate; apices 

 smooth. 



Blue clay. Along the coast. 

 PL 12, Figs. 12, 13, 14. 



TRACHYSPHENIA PETIT (1877) 

 (trachys, rough, and sphen, a wedge) 



Frustules rectangular. Valve cuneiform with coarse puncta in transverse and lon- 

 gitudinal lines; pseudoraphe narrow, linear. One species only. 



TRACHYSPHENIA AUSTRALIS PETIT 



Characters of the genus. Valve small; puncta, 6 in 10 p. Allied to Dimerogramma. 

 Shark River, N. J. Rare. 

 PL 12, Fig. 15. 



SYNEDRA EHR. (1830) 

 (synedrion, a sitting together) 



Frustules adnate in small stipitate clusters or free. Valve elongate, linear or linear- 

 lanceolate; pseudoraphe distinct; costae absent. 



The genus Synedra has few distinctive characters. As Brun remarks (Diat. des Alpes 

 et du Jura, p. 122), the dilatation of the extremities and the pseudo-nodule are of little 

 value in classification, as the intermediate forms are so numerous. Fragilaria occurs in 

 very long ribbons or fasciae, Synedra in short fasciae or radiating clusters. Fragilaria is 

 seldom longer than three or four times the width, while Synedra is nearly always so. The 

 former has fine, often subtle, markings and narrow pseudoraphe, while the latter has 

 coarser punctate striae and a more distinct pseudoraphe. 



Chromatophores usually consist of two bands, one on each of the valves. Karsten 

 states that in the marine forms the chromatophores are oval or polygonal discs, each of 

 which usually encloses a pyrenoid. 



SYNEDRA ULNA (NITZSCH) EHR. 



Frustules solitary or in twos. Valve 150-250 M in length, linear or linear-lanceolate, 

 with rostrate apices; striae, 9 in 10 /u. 



Common in rivers and streams. 



PL 11, Figs. 4, 7 and 11 (?). 



Frequently interrupted in the middle. The distinction made by Wm. Smith as to the 

 presence or absence of the celitral blank space is probably not necessary, as both forms are 

 found which are otherwise identical. 



Fig. 5 represents the formation of a sporangial frustule which differs from the usual 

 form in its inflated ends prolonged into rostrate apices. Figs. 1 and 6 are sporangial 

 frustules. 



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