300 



MICROSCOPIC FORMS OP VEGETABLE LIFE. 



envelope, does not contain many forms so interesting as those 

 which have been already described ; and it will not be requisite 

 to dwell long upon it. This envelope forms a frond, that holds 

 together all the frustules which have originated in the self-divi- 

 sion of one individual ; and it may either consist of an indefi- 

 nite gelatinous mass, or rather aggregation of masses, in which 

 the frustules are imbedded (Fig. 100), or of an indefinite cluster 



Fio. 99. 



FiQ. 100. 



Fig. 99. Mastogloia Smithii : — A, entire stipes ; B, frustuJe in its gelatinous envelope ; 0— F, dif- 

 ferent forms of frustule as seen in front view; g, side view ; H, frustule undergoing subdivision. 

 Fig. 100. Mastogloia lanceolata, 



of such masses, supported by a stipes (Fig. 99), as in the genus 

 Mastogloia ; or it may have a definite shape, either globular, com- 

 pressed, or filamentous, within which the frustules either lie 

 scattered, as in Biclcieia and Berlceleyia, or in rows, as in ScMzo- 

 nema, or in bundles, as in Homaeocladia. These are all, or nearly 

 all, marine forms ; and many of them present so strong a re- 

 semblance to the smaller filamentous Algfe, that they might 

 easily be mistaken for such. Their frustules, however, when 

 taken out of their containing tubes, frequently present so near 

 an approximation, both in structure and markings, to members 

 of the preceding groups of Diatomacese, that their close relation- 

 ship to them cannot be questioned. Very strongly marked 

 varieties sometimes present themselves within the limits of 

 a single species ; thus the valves c, d, e, f (Fig. 99), would' 

 scarcely have been supposed to belong to the same specific 

 type, were they not found upon the same stipes. The careful 

 study of these varieties, in eveiy instance in which any disposi- 



