or variously lobed to form dichotomous or trichotomous spine-tipped 

 processes; chloroplasts one to many parietal discs or plates; 

 pyrenoids usually present. 



Key to the Species 



1. Angles or poles of the cell smooth or tipped with spines 



but not extended and produced to form processes 2 



1. Angles of the cell extended and produced to form short or long 



arms or processes, branched or unbranched 19 



2. Angles tipped with 1-3 spines 3 



2. Angles smooth, or with a blunt papilla... 16 



3. Angles of the cell all in one plane 4 



3. Angles of the cell not all in one plane 9 



4. Cells crescent-shaped T. lunula 



4. Cells not crescent-shaped 5 



5. Cells 3-angled T. trigonum 



5. Cells more than 3-angled 6 



6. Cells 5-angled T. caudatum 



6. Cells 4-angled 7 



7. Cells deeply constricted on 2 sides, each of the 



four lobes tipped with a spine.— ...T. arthrodesmiforme 



7. Cells not deeply constricted on 2 sides 8 



8. Cells tetrahedral, margins concave T. regidare 



8. Cells quadrangular, margins straight T. quadraium 



9. Cells broadly ellipsoid in outhne but with 3 scarcely evident 



angles, each tipped with a short spine T. ohesum 



9. Cells polyhedral or pyramidal, with 4-5 angles 10 



10. Cells with 4 angles _ 11 



10. Cells with 5 angles T. pentaedricum 



11. Cells pyramidal _ T. regulare 



( see var. torsum ) 



11. Cells lobed, the lobes contorted, usually not all in the same plane 12 



12. Cell wall verrucose T. verrucosum 



12. Cell wall smooth 13 



13. Angles of the cell tipped with 2 spines T. bifurcatum 



13. Angles of the cell with 1 spine 14 



14. Lobes of the cell long and gradually tapering, tipped with a 

 long, slender spine, the lobes forming 2 semicells and cruciately 

 arranged, 2 in one plane and 2 in another T. Victoriae 



14. Lobes of the cell short and abruptly tapering, all in different planes 15 



15. Lobes twisted over one another, the angles 



furnished with a long spine T. arthrodesmiforme var. 



( compare with T. Victoriae ) 



15. Lobes shorter and not twisted over one another; 



angles tipped with a short spine T. regulare 



16. Cells regularly or irregularly pyramidal 17 



16. Cells not pyramidal; triangular or quadrangular 18 



17. Cells regularly pyramidal, symmetrically 4-lobed T. tumidulum. 



17. Cells irregularly 5- or 6-lobed T. gigas 



18. Cells triangular T. muticum 



18. Cells quadrangular T. minimum 



[261] 



