388 REV. EUGENE O^MEARA. 



" Navicula having on the side view longitudinal lines inter- 

 rupted in the middle without central nodule, nestling in a 

 gelatinous mass." The indefiniteness of this description is 

 not removed by the illustrative figures of the forms. Sub- 

 sequently the last-named author amended the genus and thus 

 described it: — "Frustula navicularia solitaria vel geminatim 

 — conjuncta, libera vel in muco amorpho nidiilantia, valvis 

 elliptico-lanceolatis nodulo central! terminalibusque desti- 

 tutis linea media medio interrupta " {' Flora Europ. Alg.,' 

 sect. 1, p. 227). It is doubtful whether the character ex- 

 pressed in these words, " nodulo ccntrali terminalibusque 

 destitutis," really applies to the forms Rabenhorst embraces 

 in the genus ; but if it be correct, it can scarcely agree with 

 Pfitzer^s description of the genus Frustulia as adopted by him, 

 the characters of wliich are the strong longitudinal lines 

 placed on either side of the median line, and the very peculiar 

 form of the nodules. The arrangement of the internal con- 

 tents of the cell is in the main the same as in that of the 

 Navicula, but distinguished by this peculiarity, that the 

 endochrome plates on either side in the middle of the cell 

 are pushed out from the cell-walls by the interposition of a 

 half-spherical mass of plasm. The division of the endochrome 

 plates in the case of N. saxonica occurs by fission from the 

 ends throughout, without any movement of the plates within 

 the cell. 



The genus as described by Pfitzer has the merit of possess- 

 ing very distinctive and easily recognisable characters ; it is 

 nearly equivalent to the group of Naviculce which Grunow 

 names Crassinerves, as also to the genus Vanheurckia, adopted 

 by De Brebisson. 



As regards the auxospores in this genus, the circumstances 

 are the same as in the Naviculce. 



Colletonema and Schizonema. — Colletonema vulgare, as re- 

 gards the structure of the cell, very closely approximates 

 Frustulia. A central granular plasma-mass and two en- 

 dochrome plates lying on the girdle-bands are observable 

 here, as are also the two masses of plasma interposed in the 

 middle, between the cell-wall and the endochrome plates. 

 Division of the endochrome plates is accomplished by fission 

 from the ends Avithout any movement of the plates. Other 

 species of Colletonema, C. neglectum, and C. eximium, are 

 more akin to Navicula and Pleurosigma ; and Pfitzer con- 

 sequently suggests that the genus Colletonema should be 

 divided, or, better still, the species it embraces included in 

 Frustulia and Pleurosigma. 



The only species of Schizonema which Pfitzer has had the 



