10 



p. T. CLEVE, SYNOPSIS OF THE NAVICDLOID DIATOMS. 



I have no decided opinion as to the distantly related genera PUuroneis and Achnanthes 

 (sensu stricto). They seem in some way to be akin to the peculiar Alloioneis Castracanei Pantoc., 

 which I know only from the figures. The structure resembles that of Mastogloia Grevillei. It 

 may be possible that Alloioneis Castracanei connects AcJmanthes and Mastogloia. The latter genus 

 comprises forms with very different structure, which in some cases resembles that of the Nav. ortho- 

 stichee in others that of the Nav. decussates and Pleurosigma. Some forms of Mastogloia have the 

 central nodule dilated into horns or connected with lateral areas as in the Nav. lyratce. It might 

 therefore be said that the Mastogloice belong to diff'erent types, but to prove this is not possible. 



It is not easy to find in a natural system the place a form occupies, as the most important 

 characteristics are usually the least striking. For practical purposes it is necessary to have an 

 artificial classification in order to identify rapidly an vmknown form, but it is by no means an 

 easy task to construct an artificial key of such numerous and variable forms as the naviculoid 

 diatoms. Still I have tried to compile an artificial key, which, however little rigorous it may be, 

 will still be useful. 



Artificial key of the groups and genera of naviculoid diatoms. 



■{ 



Both valves similar .... 



— — dissimilar 



„ ( Frustules with a loculiferous rim 



\ — without — — 



„ / Valves symmetrical or almost so 



\ — asymmetrical — 



. / Valves with longitudinal lines . 



I — without — — 



, / Median line sigmoid . . . . ... 



' ' \ — ■ — straight . . 



„ ( Axial part of the valve elevated into a keel 



\ — — not — — 

 _ J Structure double ... 



\ — simple . 

 „ J Central nodule with horns, enclosing the median line 



■ \ — — without — — — 

 „ j Central nodule stauroid 



\ — not — . .... 



12 

 13 



^. ( Striae distinctly punctate 

 \ — not — — 



^^ I Stria distinctly punctate 

 \ — not — — 



( Valvi 



. . 3. 

 AchnanthecB. 

 . Mastogloia. 

 3. 

 4. 

 38. 

 5. 

 13. 

 6. 



Valve with elevated ridges . ... 



without — 

 Structure double .... 



simple . . 



... j Interior stratum with transverse costse . 



' \ — — reticulate — 



.._ j Reticulum of rounded meshes . . 



\ — rhomboid or rectangular — 



„ j Striaj apparently smooth . 



( — distinctly punctate or lineate 

 1 Valve more or less linear . 

 ■ \ — — lanceolate . 



,„ j Median line sigmoid 



\ — not — ... 



f. ( Striai decussating . . .... 



\ — transverse and longitudinal 



Amphiprora. 



. . 7. 

 ScoUotropis. 

 Scoliopleura. ' 

 Diploneis. 

 9. 

 . 10. 

 11. 



Pseudo-amphiprora. 

 Caloneis (partim). 

 . . . 12. 

 Caloneis (partim). ^ 

 Cymatoneis. 

 . Neidium. 

 . . . 14. 

 . . 16. 



. . 15. 

 . Dietyoneis. 

 Trachyneis. 

 ... 17. 

 . . 18. 

 . Pinnularia. 

 Nav. lasvistriatce. 



19. 

 . . . 20. 

 Pleurosigma. 

 . Gyrosigma. 



Caloneis stauropJiora. 



Nav. luxuriantes, Nav. nicobaricce. 



