10 P. T. CLEVE, SYNOPSIS OF THE NAVICULOID DIATOM.S. 



I have no decided opinion as to the distantly related genera Flenroneis and Achnanfhcs 

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

 wliieh I know only from the figures. The structure resembles that of MnstogJoia GreiiUm. It 

 may be possible that Alloioneis Castracanei connects Achnanthes and MastogJoia. The latter genus 

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

 stichcB in others that of the Nav. decussatw and FJeitrosignia. Some forms of Mastogloia have the 

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

 therefore be said that the Mastogloia' 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 have an 

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

 easy task to construct an artificial key of sueh 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. 



I Botli valves similar 2. 



\ — — dissimilar Aclinanthece. 



- j Frustules witli a loculiferous rim Mastogloia. 



"' \ — without — — 3. 



„ I Valves symmetrical or almost so 4. 



o. 



I Val ve 



■M 



asymmetrical — 38. 



Valves witli longitudinal lines 5. 



witlinut — — 13. 



Meiliau line sigmoiil 6. 



— straight 8. 



I Axial part of the valve elevated into a keel Amphiprora. 



^- \ - - not - - 7. 



_ j Structure double Scoliotropis. 



\ — simple Scoliopleura.'^ 



„ J Central nodule witli horns, enclosing the median line Diploneis. 



' \ — — witliout — — — 9. 



„ J Central nodule stauroid 10. 



^1 - not — 11. 



, j Strife distinctly punctate Pseudo-amphiprora. 



\ — not — — Caloneis (partim). 



e distinctly punctate 12. 



not — — Caloneis (partiml. ^ 



^j, f Valve with elevated ridges Cymatoneis. 



.., I Strise distinctly punctate 12 



2 i 



I — without — J^eidimn 



13. jStv 



Structure douhle Ii. 



simple 16. 



.,, I Interiör stratum with transverse costa -. Mastoneis. 



\ — — reticulate — 15. 



.., I Reticulum of rounded meshes Dictyoneis. 



\ — rhomboid or rectangular — Trachyneis. 



P I Stria) apparently smooth 17. 



I — distinctly punctate or lin 



I Valve more or less linear Pinnularia. 



ineate 18. 



i 



\ — — lanceolate Nav. Iceri.^triatce. 



,„ /Median line sigmoid ..'. 19. 



^^- i - not - 20. 



..„ ( Stri» decussating Ple%irosig>iia. 



— transverse and longitudinal Gyrosigma. 



' Caloneis staurophora. — ' Nav. luxuriantes, Nar. nicoharicte. 



