THE DIATOMACE.E OF PHILADELPHIA AND VICINITY 115 



3. Apiculatce. Keel very near the edge; valve linear or somewhat narrower in the 

 middle; strise on the longitudinal fold fainter than on the remaining surface, or wanting; 

 puncta not in quincunx. 



4. Pseudo-Tryblionella. Keel more or less close to the edge; valve with a more or less 

 deep longitudinal fold over which the strise are spread in the same way as over the remain- 

 ing surface; keel puncta always distinct. 



5. Circumsutce. Valve with more or less wide longitudinal fold; keel very excentric; 

 keel puncta quite evident; surface of valve irregularly punctate and also traversed by rows 

 of delicate puncta which belong to a different layer of the valve. 



6. Dubice. Like the group Pseudo-Tryblionella, but the valves are not so much 

 folded; frustules sometimes narrowed in the middle. The separation of species is difficult 

 and, in part, doubtful. Keel excentric. 



7. Bilobatce. Like the group Dubise, but with more central keel and so forming a 

 transition to the group Pseudo-Amphiprora; valves without longitudinal folds. 



8. Pseudo-Amphiprora. Valve with quite central, sharp keel, arcuate, without longi- 

 tudinal fold; keel puncta always evident; frustule narrowed in the middle with more or 

 less marked central nodule. 



Includes two species not found in this locality. 



9. Perry a. Valve arched with very sharp central keel; not narrowed in the middle; 

 keel puncta mostly on short or long lines which are sometimes interrupted. 



Includes six species not found in this locality. 



10. Epithemioidece. Keel excentric; keel puncta extended into costse across the 

 entire valve. 



11. Grunowia. As in the group Epithemioidese, except that the cost are shorter, 

 not extending across the valve; keel very excentric. 



12. Scalares. Like Grunowia, but with sharper, somewhat excentric keel; transverse 

 section of frustule quadrangular. 



13. Insignes. Like Scalares, but with more central keel so that many of the forms 

 are near the group Perry a; frustule somewhat sigmoid. 



14. Bacillaria. Keel central or nearly so; valve somewhat arched; keel sharp, as in 

 the group Insignes. 



15. Vivaces. Keel moderately excentric; valve, according to position, semi-lanceolate, 

 with keel puncta in short rows, or lanceolate with quite central keel. The valves have in 

 many positions a resemblance to Hantzschia, so that N. vivax frequently becomes con- 

 founded with a form of H. amphioxys. The median keel puncta are not distant and a 

 central nodule is not evident as is the case in all species of Hantzschia. 



16. Spathulatce. Like the group Bacillaria, but usually with very delicate striated 

 valves; keel in valve view usually bordered with two parallel lines. 



17. Dissipates. Like Vivaces and Spathulatse, but with smaller central keel and without 

 parallel lines. Valves usually small, very delicately striated; no central nodule. 



18. Sigmoidece. Keel quite central; no parallel lines; frustule sigmoid; valve without 

 longitudinal furrow; keel puncta not extended; no central nodule evident. 



19. Sigmata. Like Sigmoideae, but with a more excentric keel. 



20. Obtusce. Like Sigmata, with a more or less excentric keel which has in the middle 

 a small bending to the inside; middle keel puncta somewhat more distant than the others, 

 and between them a central nodule evident. 



