THE DIATOMACE^E OF PHILADELPHIA AND VICINITY 55 



NAVICULOIDE^ 



In discussing the Naviculoid group, the general divisions of Cleve are here followed, 

 and all diatoms having a true raphe are included. I have added the genus Epithemia and 

 also Rhopalodia, partly because they contain a raphe of a certain kind and partly because 

 they resemble the markings of certain of the genus Hantzschia in the following group, 

 although in other respects there is probably no similarity. 



The difficulty of combining the numerous genera into groups which are naturally 

 affiliated is avoided in the following arrangement based on superficial similarities, and is 

 intended merely as an artificial key. To unite all forms having a raphe and which are 

 symmetrical with valves similar and not sigmoid, under the one genus Navicula, as has been 

 the custom previous to the publication of Cleve's monograph, would result in associating 

 species differing in so many respects in relation to structure of the valve and cell contents 

 that it seems advisable to retain the new genera, especially as the original genus is likely to 

 be still further reduced when more is known of the structure and life history of the group. 



KEY TO THE GENERA 



Valves dissimilar. Achnanthese 



symmetrical Cocconeis 



asymmetrical 



to the longitudinal axis Anorthoneis 



to the transverse axis Rhoicosphenia 



in zone view Achnanthes 



Valves similar and asymmetrical 



asymmetrical to the longitudinal axis 



valves parallel Cymbella 



valves not parallel Amphora 



valves keeled, twisted (sometimes symmetrical) Amphiprora 



valves keeled Tropidoneis 



valves reniform and keeled Auricula 



median line sigmoid at the ends Scoliotropis 



asymmetrical to the transverse axis 



striae punctate and costate Gomphoneis 



striae punctate Gomphonema 



Valves similar, symmetrical and sigmoid 



striae oblique Pleurosigma 



striae at right angles Gyrosigma 



Valves similar, symmetrical, not sigmoid 



striae punctate, nodules elongated Frustulia 



striae subtly punctate, central nodule forked Amphipleura 



striae punctate and reticulate, in two strata Dictyoneis 



striae punctate and alveolate, in three strata Trachyneis 



striae punctate, in two strata Brebissonia 



striae interrupted by blank lines Anomceoneis 



