DIATOM ACEiE. SCHIZOXEMEiE 1 MASTOGLOIA. 



309 



of nearly equal diameter throughout, within which the frustules 

 lie either in single file or without any definite arrangement 

 (Fig. 141) ; all these frustules having ai'isen from the self-division 

 of one individual. In the genus Mastogloia, which is specially 

 distinguished by having the annulus furnished with internal 

 costse projecting into the cavity of the frustule, each frustule 



Fig. 142. 



Fig. 143. 



Fig. 142. Mastogloia Smithh :— a, entire stipes ; b, frustule in 

 its gelatinous envelope ; c— F, different forms of frustule as 

 seen in side view ; g, front view ; h, frustule undergoing sub- 

 division. 



Fig. 143. Mastogloia lanceolata. 



separately supported on a gelatinous cushion (Fig. 142, b), 

 which may itself be either borne on a branching stipes (a), or may 

 be aggregated with others into an indefinite mass (Fig. 143). The 

 careful study of these composite forms is a matter of great im- 

 portance ; since it enables us to bring into comparison with each 

 other great numbers of frustules which have unquestionably a 



