\i 



are straight to the ends; the side view of the CVw///m/wm- in 

 group III is circular and the front view rectangular. Though 



O <" 



these serve as types, yet each group has many except ional forms : 

 thus the frustules of Cymbella appear more or less swollen in a 

 front view, while the side view shows one valve more swollen 

 than the other. The frustules of Gomplionema are cuneate on 

 side view and more or less irregularly swollen in front view. 

 The frustules of Meriiliox and L'liiiiiilophera are also cuneate in a 

 greater or less degree. 



The absence of the raphe or mediam line from one of the 

 valves, is another departure from the normal form in group I. 

 The frustule is then curved with the raphe and central nodule, 

 on the concave side (valve) as in Achnanthes, Acliiniiil/iiili.inii and 

 Cocconeis. The obliteration, partial or entire, of the central 

 nodule as in StannnK'ix. is another variation in group I. In 

 some of the Xavicule.e the median line is more or less sigmoid. 

 not straight; of these. Grunow made the genus Nro//o/>/r///Yc . 

 also, the Pleurosigma differ from the Xaviculca' in the sigmoid 

 median line as well as in the sigmoid form of the frustules. 



As a general rule, free forms belong to group I. the Hat fila- 

 mentous to group II. the cylindrical and most of the irregular 

 forms to group III. 



CONTENTS OF DIATOMS. 



The colored internal portions of a Diatom are for the same 

 genus, usually arranged in the same manner. Any departure 

 therefrom, is accompanied by differences in the build of the 

 frustule: for which reason, microscopists have been disposed 

 to classify Diatoms agreeably to the arrangement of the endo- 

 chrome ; which, however, has nothing to commend it especially 

 as regards the immense masses of fossil Diatoms which can be 

 described only by their frustules. 



The contents of the Diatom are enclosed in a membrane that 

 is firmly attached to the frustule, and possesses a higher vitality 

 than the external membrane; for when the contents thrust the 

 halves of the tVustule apart, the internal membrane elaborates 



the si lex for the inside edge of the sutural / ;inl the /.ones 



for the new valves. 



Within the membranous sac is a dimly colored band more or 

 less granular, attached to the inner surface of the internal mem- 

 brane directly beneath the valves and along the /.ones; it is 

 contractile and apparently serves to hold the valves togriher 

 besides equally dividing the contents of the Diatom, by means 

 of extremely thin membranous walls which issue from it. 



