The microscope and its revelations . nus Stauroneis,which belongs to the same group, differs from all the preced-ing forms, in having the central nodule of each valve dilatedlaterally into a band free from strise, which forms a cross withthe longitudinal band; of this very beautiful form, some speciesare fresh-water, others marine; and the former present them-selves frequently in certain infusorial earths,* 185. The group we have next to notice, consists of thosegenera which have the frustules, after self-division, attachedby a gelatinous cushion, or by a dichotomous stipes, (Fig. 89). _ * For
The microscope and its revelations . nus Stauroneis,which belongs to the same group, differs from all the preced-ing forms, in having the central nodule of each valve dilatedlaterally into a band free from strise, which forms a cross withthe longitudinal band; of this very beautiful form, some speciesare fresh-water, others marine; and the former present them-selves frequently in certain infusorial earths,* 185. The group we have next to notice, consists of thosegenera which have the frustules, after self-division, attachedby a gelatinous cushion, or by a dichotomous stipes, (Fig. 89). _ * For some very cnrious examples of the extent to which variation in form,size, and distance of strise, may take-place in this group, among individualswhich must be accounted as of the same species, see the memoirs of Smith and W. Gregory ab-eady referred-to (p. 313, note). DIATOMACE^ :—STIPITATE FOEMS. 323 Many of these present a strong resemblance to some of thepreceding, so far as the structure of their frustules is itself. Gomphonema geminatum: its frustules connected by stipes. concerned; so that when, as frequently happens, they arefound unattached, the difference is not apparent. In Synedra(Eig. 102, /), which is not unlike a long narrow Navicula (animperfect longitudinal line, with central and terminal dila-tations, being often, but not always, apparent), the frustulesare at first invariably attached to larger Algse, or otheraquatic plants, by a cushion-like gelatinous basis; and whenthey remain adherent to this after repeated subdivision, theysometimes form a fan-like band of frustules, not unlike that oft3 324 MICROSCOPIC rOEMS OF VEGETABLE LIFE. Lichmophora (Fig. 91), or even a stellate cluster, the appear-ance of which is extremely characteristic. So again, thefrustules of Gomphonema (Fig. 90) in a side view, are not Fia. 90.
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