222 PROCEEDINGS OF THE 



the valve to the other. The only species in which the strise are 

 not pervious is D. ocellata. 



A. Frustules not ocellate on F. V. costse attached to the alternate 

 angles of the same frustule. 



1. D. deciinens A. & R. — costse reduced to mere points on the 

 margin; striae very conspicuous — pervious. 



In this species the alternate strise appear, under a high power, 

 to be coarser or stronger than the others. From the costae being 

 reduced to mere puncta, this might be referred to Nitzschia, but 

 tlie puncta are on the margin, not an intramarginal angle or keel, 

 and the zone is therefore parallel to the valve. It is more allied 

 to some species of Tryhlionella, but there is no ridge (or depres- 

 sion) along the middle. 



2. D. ohtusa — costae (strengthened stria) tapering from a stout 

 base to a fine point, and extending to the other margin; striae 

 conspicuous — pervious. This is D. ohtusa, Sm., but the figure is 

 imperfect. It is not Kiitzing's species, which is now understood 

 to be Diatoma vulgare. Gruuow has given good figures of it, under 

 the new name of Dent. Kutzingii, in the Wien. Verb, for 1862, 

 tab. xii., f. 15 and 27. The frustule varies much in size and 

 form. 



3. D. sinuata — costae equally strong, reaching to about the 

 middle of the valve; striae fine, but conspicuous — pervious. 



This species is extremely variable in size and form, occasionally 

 losing entirely the sinuate margin of the valve, which gave rise to 

 the specific name. It is a true Denticula, and not a species of 

 Dimeregramma, as proposed by M. Ralfs. D. tahellaria of Grunow 

 appears to be one of the many variations. Rabenhorst 1ms most 

 unnecessarily formed for the two a new genus called Gnmowia. 



B. Frustules ocellate on F. V. ; ocelli and costae attached to the 

 adjacent angles of the same frustule. 



4. D. tenuis — ocelli conspicuous; costae pervious, strong, and 

 equally so throughout; striae conspicuous — pervious. 



5. D. inflata — same as in the last, but strise very faint. 



These two ought probably to be united. Both present the 

 same variations in the size and form of the frustule and valve. 

 The latter is sometimes extremely narrow, even more so than is 

 represented in Smith's figure, and sometimes almost oval, although 

 usually with acute extremities. AVithout seeing authentic speci- 

 mens, it is almost impossible to say to which Kiitzing's species 



