10 p. T. CLEVE, SYNOPSIS OF THE NAVICULOID DIATOMS. 



I have no decided opinion as to the distantly related genera Flcuroncis and Adinnnthcs 

 (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 Mnstojloin GrcviUei. It 

 may be possible that Alloioneis Castracanei connects Achncinthcs and Mastoyloia. The latter genus 

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

 stichtp, in others that of the Nav. decussaUv and FJenrosigma. 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 different 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 liave an 

 artihcial classification in order to identify rapidly an unknown 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, hoAvever little rigorous it may be, 

 will .still be useful. 



M 



Artificial knj of the groiips and genera of naviculoid diatoms. 

 Both valves .similar 2. 



— dissimilar Achnanthece. 



jj I Frustules with a loculiferous rim Mastogloia. 



~'' \ — without — — 3. 



Valvus svmniotrical or almost so 4. 



3 



a.symiiietrical — 38. 



. I Valves with longitudinal lines 5. 



\ — without — ■ — 13. 



J. J Jledian lini' sigmoid G. 



1 — — straight 8. 



I Axial j)art of the valve elevated into a keel Amphiprora. 



^- \ - - not - - 7. 



Structure double Scoliotropis. 



{ 



I 



Central nodule with horns, enclosing the median line Diploneis 



simple Scoliopletira.^ 



without — — — 9. 



„ J Central nodule stauroid 10. 



" \ — not — 11. 



j Stri:e distinctly punctate Pseudo-aniphiprora. 



\ — not — — Caloneis (partim). 



_. I Striae distinctly punctate 12. 



\ — not — ^ Caloneis (partim).^ 



^^ j Valve with elevated ridges Cymatoyieis. 



\ — without — Neidium. 



^., ( Structure double 14. 



\ — simple 16. 



^ . I Interior stratum with transverse costa; Mastoneis. 



\ — — reticulate — 15. 



^j. j Reticulum of rounded meshes Dictyoneis. 



' \ — rhomboid or rectangular — Trachyneis. 



^,. I StriiB apparently smooth 17. 



\ — distinctly punctate or lineati; 18. 



^^ I Valve more or less linear Pinnnlnria. 



It. ' 



J Valve 1 



— lanceolate Xar. Iwristriatcp. 



. I Median line sigmoid 19. 



\ — not — 30. 



j,j I Striin decussating Pleurosigma. 



\ — transverse and longitudinal Gyrosigma. 



* Caloneis staurophora. — ^ Nai\ luxuriantes, Nav. niioharicce. 



