X DIATO^IACK.K <>K NORTH AMKUK'A. 



STRUCTURE OK DIATOMS. 



Iii the pill-box-like silicious structure of Diatoms, the top 

 and bottom constitute the win-*: the sides are known as the 

 connecting membranes or sutuntl -ones and when detached are 

 termed hoops; in some, the valves do not fold over but merely 

 rest against each other edge on edge. The line of junction 

 forms a suture, along which the valves readily separate. H. L. 

 Smith divides Diatoms into three groups, according to the pres- 

 ence or absence of a raphe or median line, viz., Raphidiese, 

 Pseudo-Eaphidieie and Crypto Raphidie;e. The first has a dis- 

 tinctly true raphe or median line which usually has median and 

 terminal enlargements or nodules ; the second has no raphe but 

 a pseudo-median line without nodules or enlargements ; in both 

 of these groups, the valves are much longer in the direction of 

 the raphe or pseudo-raphe and therefore are more or less bacillar. 

 The third group has the raphe concealed and the pseudo-raphe 

 absent, the valves circular, often angular and sometimes broadly 

 elliptical. 



The raphe is a true cleft which in the first group divides the 

 valves equally and is supposed to be the means by which the 

 contents of the frustule communicate with the outer world. In 

 one or the other of these three groups, all known forms of the 

 Diatom may be included. 



Besides the simple frustule and envelopes, diatoms often pre- 

 sent hairs, horns or bristles, which are usually silicious. They 

 are most abundant on group three, less so on group two and are 

 rarely found in group one. They are mostly marginal or sub- 

 marginal, and are sufficiently persistent to have led to the nam- 

 ing of genera and species in accordance with their presence or 

 absence. 



The mere adherence of the frustules gives rise to what are 

 termed filamentous forms ; thus, in group I, we have Diadesmix. a 

 genus founded alone on this characteristic ; Fragilar/a, in group 

 II, is but an association of N///<rrfm,' in the form of a straight 

 filament, and in group III Meloxint is a straight filament of 

 frustules no way different from Cyclotilla or even Coscinodt.vitx 

 except in cohering more' or less firmly after the self-division of 

 the frustules. 



The typical forms of the three groups are regular in outline ; 

 thus, a \<tr!cula from group I is in side view (the valve* more 

 or less inflated and rectangular in front ; the side view inSynednr 

 of group IT presents straight, parallel margins, the ends drawn 

 more or less together; in the front view, however, the margins 





