36 TUFTS COLLEGE STUDIES, VOL. IV, No. 7 



3. Sides straight to snbconvez, membrane reticulate. 



6. T. reticulalutn. 

 3. Sides straight to sinuate, membrane smooth or finely punctate. 



2. T. mti tic it i n. 



4. Cells with 4 angles. 5. 



4. Cells with 5 angles or more. 7. 



5. Cells 18-30 Mdiam. ; membrane granular-punctate. 5. T.punctatum. 

 5. Membrane smooth. 6. 



6. Cells 6-1 1 M diam., sides distinctly emarginate. 4. T. minimum. 



6. Cells 15-21 M diam., sides not emarginate. 3. T. tetragonum. 

 7. Irregularly 5-6-angled, 35.75 M diam., menbrane thin. 13. T.gigas. 

 7. Rather regularly 6-8-angled, 17-26 M diam., membrane laminate. 



ii. T. pachyderm u in . 



8. Cells semicircular to ovate with sides of unequal curvature, a spine 

 at each end. 18. T- obesum. 



8. Cells not as above. 9. 



9. Each angle with one spine. 10. 



9. Bach angle with 2 or more spines. 17. 



10. Spines short. 11. 



10. Spines longer. 16. 



u. Cells asymmetrical or contorted. 29. 



n. Sides and surfaces of cells symmetrical or nearly so. 12. 



12. Cells flattened, triangular. i. T. trigonum. 



12. Cells more than 3-angled. 13. 



13. Cells 4-angled. 14. 



13. Cells 5-angled. 15. 



14. Cells flattened, quadrate. 7. T. quadratuin. 



14. Cells tetraedral. 12. T. regular e. 



15. Cells flattened. 9*. T. caudatum. 



15. Cells pen taedrical. 20. T. pentaedricutn. 



16. Cells irregularly triangular, twisted, 73-81 M diam., spines about 

 30 M long. 19. T.tortittn. 



16. Cells either fusiform, 65 X 12 M, or stellate with 3 rays, body about 

 36 M diam., margins sinuous, angles passing gradually into 

 spines. 27. 7\ proteifornif. 



17. 4- 10 slender spines, 30-40 M long, at each angle. 21. '/'. */>/// A u/<;. 

 17. Spines relatively shorter and stouter. 18. 



18. Cells with arm-like prolongations. 19. 



18. Cells without arm-like prolongations. 23. 



19 Arms not divided, except at tip. 20. 



19. Arms 1-5 times divided. 21. 



20. Cells tetraedral. 22. 



20. Cells flatter. 28. 



21. Ultimate division* very slender, acute. 8. T. gracile. 



it. Ultimate divisions broader, especially in top view. 30. 



