MICROSCOPIC FORMS OF VEGETABLE LIFE. 



297 



tion of their length, often more conspicuously than in the example here 

 represented. This family contains free, adherent, and stipitate forms; 

 one of the most common of the latter being the Achnanfhes longipes 

 (Fig. 185), which is often found growing on Marine Algae. The differ- 

 ence between the markings of the upper and lower valves is here distinctly 

 seen; for while both are traversed by striae, which are resolvable under a 

 sufficient power into rows of dots, as well as by a longitudinal line which 

 sometimes has a nodule at each end (as in Navicula), the lower valve (a) 

 has also a transverse line, forming a sturos or cross, which is wanting in 



186, 



Rhizosolenia Achnantheslongipctf: a, 6, 

 imbricata. c, d, c. successive frustules in 

 different stages of self-divi- 

 sion. 



Qomphonema geminatum: its frus- 

 tules connected by a dichotomous 

 stipes. 



the upper valve (e). A persistence of the connecting membrane, so as to 

 form an additional connection between the cells, may sometimes be ob- 

 served in this genus; thus in Fig. 185, it not only holds together the two 

 new. frustules resulting from the subdivision of the lowest cell, , which 

 are not yet completely separated the one from the other, but it may be 

 observed to invest the two frustules, b 'and c, which have not merely sep- 

 arated, but are themselves beginning to undergo binary subdivision; and 

 it may also be perceived to invest the frustule d, from which the frustule 

 e, being the terminal one, has more completely freed itself. In the 

 Family Cytribellece, on the other hand, both valves possess the longitu- 

 dinal line with a nodule in the middle of its length; but the valves have 

 the general form of those of the Eunotiece-, and the line is so much 

 nearer one margin than the other, that the nodule is sometimes rather 



