182 TUFTS COLLEGE STUDIES, VOL. II, No. 3 



A genus of marine and fresh water species, of world-wide dis- 

 tribution ; by no means easy to distinguish from species of Hor- 

 jniscia, Chactomorpha, etc., whose filaments are also of a single 

 series of cylindrical or somewhat inflated cells. The number of 

 nuclei can be ascertained only by careful examination, and 

 while the shape of the chromatophore is of a different type in 

 each genus, the variations from the type are many and perplex- 

 ing. In herbarium specimens the chromatophore characters 

 are usually indistinct. In general, Chactomorpha is firmer and 

 stiffer, and has longer cells than Ulothrix ; none of our species 

 of Ulothrix exceeds 50 /A cliam., while few species of Chaetomor- 

 pha are less than 100 /u diam. Hormiscia species also are 

 usually of larger dimensions than Ulothrix species, but there is 

 little difference in cell length or consistency. As between the 

 two genera, the only tests are the number of nuclei, requiring 

 pretty careful study, and the form of the zoospores, ascertain- 

 able only occasionally. As regards dried specimens, only per- 

 sonal acquaintance with the various species, or comparison with 

 authentic specimens, is of any use. 



KEY TO THE SPECIES OK ULOTHRIX. 



i. Marine or brackish. 2. 



i. Fresh water. 5. 



i. Thermal. 2. U. caldaria. 



2. Occasionally branching. 12. U. laetevirens. 



2. Always simple. 3. 



3. Cells usually much shorter than their diameter. 10. U. flacca. 

 3. Cells usually longer than their diameter. 4. 



4. Chromatophore a more or less complete ring at the middle of the 

 cell. 9. U. implexa. 



4. Chromatophore a curved lateral disk, extending from the pyrenoid 



in all directions, but not filling the cell. n. U. subflaccida. 

 5. Filaments not over 9 M diatn.; pyrenoid single. 6. 



5. Filaments 1 1-45 /x diam.; pyrenoids several. 7. 



6. Filaments 7-9 M diam. 3. U . tcnarima. 



6. Filaments 5-6 /j. diam. i. U. rariabilis. 



7. Mature filaments 25-45 M diam. 8. U. zonata. 



7. Filaments not over 20 fj. diam. 8. 



8. Filaments torulose. 6. U. moniliforntis. 



8. Filaments not torulose. 9. 



9. Cells 1-2 diarn. long. 7. U. aequalis. 



9. Cells not over \ diarn. long. 10. 



