26 BRITISH DIATOMACE-ffl. 



which sufficiently distinguishes this and the following genus ; while 

 the presence of a stipes of more or less perfect development sepa- 

 rates Achnanthes from its ally. The peculiar form, conspicuous size, 

 and general diffusion of the several species of the present genus 

 place them among the most singular and interesting of the Dia- 

 tomaceae, and have attracted the attention of all observers ; still but 

 very little is known of the circumstances which control their de- 

 velopment, and conjugation has been noticed in one species only, 

 and that by myself alone. The anomalous form of the valves, and 

 the exceptional character of their markings, I am wholly unable to 

 explain ; the additional strength given to the lower valves by the 

 presence of nodules and a transverse band of silex being apparently 

 uncalled for by any peculiar stress or demand upon this portion of 

 the frustule. 



Some confusion has been introduced into the genus by adopting 

 the length of the stipes as a specific character. This feature, though 

 constant in some, varies greatly in other species, and ought not there- 

 fore, in the latter cases, to be regarded as an important distinction ; 

 its comparative length or even presence in such species wholly de- 

 pending upon the stage of growth at which the filaments have arrived. 

 Nor is the length of the filament, into which the united frustules 

 cohere, a satisfactory criterion, the coherence being apparently de- 

 pendent upon accidental circumstances, and varying with the habitat 

 and season. The form and striation of the valve are more certain 

 and constant characters, and these, though subject to slight modifica- 

 tions, are our best guides in the discrimination of species. The 

 habit of growth in the stipitate species will also be found an im- 

 portant aid, the filaments being sometimes scattered over the larger 

 Algae to which they are affixed, sometimes springing from a point 

 around which their stipes cluster in considerable numbers. It will 

 be seen from the figures given, that the connecting membrane in the 

 present genus is usually delicately striated, a character which has 

 been before noticed in Himantidium, and will be found in others of 

 the filamentous Diatomaceae. 



1. Achnanthes longipes, Ag. Filament of few frustules, from 2 to 

 16; V. linear-elliptical, obtuse, with a slight central constriction, 

 costate ; costae unequally distant, from 6 to 12 in *00l" ; striae 



