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



3. Cells without polar bulbs or thickenings. 4. 



3. Cells with polar bulbs or thickenings. 5. S. incrassatulus. 



4. Cells all in one plane. 5. 



4. Cells not in one plane. 6. 



5. All cells straight. I. 5. obliqmts. 



5. Median cells straight, end cells curved. 2. 5. dimorf>hns. 



6. Cells alternate, end cells usually not in same plane as others. 



3. 5". Bernardii. 



6. Cells forming a curved surface. 4. S. acuminatus. 



7. Cells in a linear or alternating, not a double series. 6. S. bijuga. 

 7. Cells in two series. 7. 5. arcuatus. 



8. Cells with teeth at poles, but no other projections. 



8. S. dcnticulatus. 



8. Cells with projections other than teeth. 9. 



9. Cells with longitudinal ridges. 10. 



9. Cells without ridges. 12. 



10. Cells with ridges only. 9. S. acutifortnis. 



10. Cells with ridges and other projections. n. 



ii. Cells with teeth at poles. 10. S. brasiliensis. 



II. End cells with spines. n. S. artnatus. 



12. Cells with short spines or granulations. 13. 



12. Cells with long spines or horns. 14. 



13. Spines or granulations covering the wall. 12. 5. Hystrix. 



13. Spines or grahulattons in longitudinal rows. 13. 5. serratus. 



14. Spines on all cells. 15. 



14. Spines on end cells only. 16. 



15. Spines both on poles and middle part of cell. 14. S. abandons. 

 15. Spines only at poles. 15. 5. longus. 



16. Cells in lateral contact throughout. 16. S. quadricauda. 



16. Cells in contact for middle third only. 17. S. opoliensis. 



1. S. OBLio.uus(Turp.) Kutzing, 1833, p. 609 ; G. M. Smith, 

 I9i6a, p. 428, PI. XXV, fig. 7; XXIX, figs. 63-68; Collins, 

 1909, p. 168 ; Achnanthes obliqua Turpin, 1820, fig. 9; 1828, 

 p. 3I 2, PI. XIII, fig. 9. 



2. S. DIMORPHUS (Turp.) Kiitzing, 1833, p. 608; G. M. 

 Smith, I9i6a, p. 434, PI. XXV, fig. 8; XXXII, figs. 185-189; 

 XXXIII, figs. 190-195 ; 5. obli(juusv*r.dimor/>hitsCo\\\\\s t 1909, 

 p. 169 ; Achanthesdimor/>hnsTurv\n t 1820, fig. 2 ; 1828, p. 312, 

 PI. XIII, fig. 12. 



3. S. BKRNAKDII G. M. Smith, I9i6a, p. 436, PI. XXV, 

 fig. 6; XXXIII, figs. 196-208. Colonies of 2-8 cells ; cells fusi- 

 form, often sigmoid, 8-16 X 3 7 M ; in a loose alternate series, 

 attached only near the poles. Wis., G. M. Smith. 



4. S. ACUMINATUS (Lagerh.) Chodat, 1902, p. 211 ; Selenas- 



