CLASSIFICATION OF DIATOMACE^l. 



289 



imbedded in gelatine, nor enclosed in a membranaceous tube ; 

 whilst the second is composed of those forms, whose frustules 

 have a gelatinous or membranaceous envelope. The frustules of 

 the Diatoms belonging to the first tribe, however, may have 

 various degrees of connection ; for whilst, in some (a), the union 

 is dissolved almost immediately upon the completion of self- 

 division, there are others (b) in which a gelatinous cushion or a 

 stipes, to which the frustules are attached by a small portion of 

 their surface, maintains a partial connection between the divided 

 frustules, and others (<?), again, in which the frustules remain in 

 more or less complete cohesion, and form filaments, which, if 

 the cohesion be limited to the angles of the frustules, are mere 

 zigzag chains, but, if the cohesion extend to the entire surfaces 

 of their sides, are continuous filaments, either flattened or cylin- 

 drical. 



181. That section of the first tribe, in which the frustules are 

 entirely disconnected from each other after the completion of 

 their self-division, includes a number of beautiful discoidal forms, 

 which seem to constitute a natural group, and may therefore be 

 appropriately noticed in connection with each other. The genus 

 Coscinodiscus is one of great interest, from the vast abundance 

 of its valves in certain fossil deposits (Fig. 101, a a a), especially 

 the Infusorial earth of Richmond in Virginia, of Bermuda, and 

 of Oran, as also in Guano. Each frustule is of discoidal shape, 

 being composed of two nearly flattened valves, united by a hoop ; 

 so that, if the frustules remained in adhesion, they would form 

 a filament resembling that of Meloseira (Fig. 97). The regu- 

 larity of the hexagonal divisions on the valves, renders them 

 beautiful microscopic objects ; in some species, the areolse are 

 smallest near the centre, and gradually increase in size towards 

 the margin ; in others, a few of the central areolse are the largest, 

 and the rest are of nearly uniform size. Most of the species are 

 either marine, or are inhabitants of brackish water ; when living, 

 they are most commonly found adherent to sea-weeds or zoo- 

 phytes ; but when dead, the valves fall as a sediment to the 

 bottom of the water. In both 

 these conditions, they were found 

 by Prof. J. Quekett in connec- 

 tion with Zoophytes which had 

 been brought home from Melville 

 Island by Sir E. Parry ; and 

 the species seemed to be identi- 

 cal with those of the Richmond 

 earth. Nearly allied to the pre- 

 ceding is the beautiful genus 

 Actinocyclus (Fig. 84), of which 

 also the frustules are discoidal in 



form, but of which each valve, instead of being flat, has an undulat- 

 ing surface, as is seen in front view (B) ; giving to the side view (A) 



19 



FIG. 84. 



Actinocyclus undulatus: A. side view; 

 B. front view. 



