DIATOMACEJE 



589 



frustule and the presence of which imparts a peculiar interest to 

 the group; not merely 011 account of the elaborately mar keel pattern 

 which it often exhibits, but also through the perpetuation of the 

 minutest details of that pattern in the specimens obtained from 

 fossilised deposits. This silicified casing is usually formed of two 

 perfectly symmetrical valves united to one another by means of two 

 embracing rings which constitute the connecting zone or girdle, and 

 thus exactly represent a minute box which serves for the reproduc- 

 tion of the 'species. This process is known as the encystmeiit, and 

 is not uncommon, especially amongst the Xariculett'. frustules being 

 frequently found amongst them o*pen from the separation of the t\vo 

 valves, showing the two rings covering each other, as the lid of a 

 box may cover a portion of the box itself. 



The following definitions of terms used in describing the siliceous 

 envelope of diatoms have been proposed by the late eminent diato- 

 mologist, Mi'. J. Deby. The radiating lines (called by some ' cost* ' 

 or ' caiialiculi ') starting from the outer margin of the valve, and 

 converging towards the interior of the disc, are rays or marginal 

 rays. They may be simple, which is most usual ; or moniliform, 

 i.e. composed of a single or double row of ' beads ; ' or infundibuliform, 

 having the outline of a funnel with a long outlet ; the upper broad 

 portion is the * funnel/ the slender part the ' stem.' The central 

 portion of the valve inside the internal termination of the rays is 

 the area- it may be smooth and hyaline, or it may be striate, or 

 simply punctate or dotted, the dots forming regular lines or else 

 being irregularly scattered. If this area becomes reduced to a 

 median linear blank space, or to a simple elongated line, it is known 

 as tin- rft/thfi or psetido-raphe. 



Dr. O. Miiller proposes the term epitheca for the overlapping 

 half-cell of the diatom, the under-lapping half-cell being the hypo- 

 theca ; for the girdle- bands he proposes the term pleura . 



In describing diatoms, the aspect in which the girdle is turned 

 towards the observer is known as the 'front' or 'girdle' view ; that 

 in which the surface of the valve is turned towards the observer is 

 the 'side ' or ' valve ' view. 



It is not correct to designate the line shown in the front view 

 of the outer ring as the line of ' suture,' since the suture is the line 

 of meeting bounding two surfaces placed on the same plane. The 

 form resulting, however, varies widely in different diatoms ; for 

 sometimes each valve is hemispherical, so that the cavity is globular ; 

 sometimes it is a smaller segment of a sphere resembling a watch- 

 glass, so that the cavity is lenticular ; sometimes the central portion 

 is completely flattened and the sides abruptly turned up, so that the 

 valve resembles the cover of a pill-box, in which case the cavity will 

 l>e cylindrical; and these and other varieties may co-exist with any 

 modifications of the contour of the valves, which may be square, 

 triangular (fig. 442), heart-shaped (fig. 454, A) ,boat-shaped (fig. 453, 

 A), or very much elongated (fig. 449), and may be furnished (though 

 this is rare among diatoms) with projecting outgrowths (figs. 458, 

 45D). Hence the shape presented by the frustule differs completely 

 with the aspect under which it is seen. In all instances, the 



