TERMINOLOGY AND CLASSIFICATION OF DIATOMS. 85 



A valve should be examined with reference to (1) its general appearance ; 

 (2) its outline; (3) its extremities (apices) ; and (4) its processes (appendices). 



1st. General Appearance. The valve is said to be smooth when 

 it displays absolutely no pattern or any marking on the surface. This 

 word, which was so frequently used by former authors having but 

 imperfect microscopes at their disposal, can only now be applied in a 

 few rare cases. 



The valve is alveolate {cellular or cellulose, according to some 

 authors), when it is marked with coarse dots, giving an hexagonal or 

 angular appearance as in the Tnceratium (pi. 21, f. 643). 



Many diatoms of the sub-family Crypto-raphidieas have an alveolate 

 structure. The valve is striate when it displays struc or fine lines 

 formed of delicate puncta, which the microscope resolves (separates) with 

 difficulty, as in Ampliipleura pellucida (pi. 5, f. 253). It is monitiform 

 when these stria are formed of rows of very distinct beads, as, e.g., in 

 Navicula Lyra (pi. 4, fig. 161). 



The valve is furnished with costtz when it displays very distinct 

 coarse lines, often thick, and resembling ribs, and cannot be resolved 

 into puncta. These costal are often the result of the confluence of 

 alveoles ; this term is, moreover, also applied to the thickened spaces 

 between certain rows of fine puncta. 



When the strias or costal are not interrupted by a median line or 

 raphe the valve is said to be pervious striate or transversely furnished with 

 costag ; it is said to be dimidiate striate or costate when the striae or 

 costaa only extend over the moiety of the valve. 



The valve is called arcuate when one of the margins is more curved 

 than the other, as in Ceratoneis Arcus (pi. 10, f. 401). The convex 

 side is often called the dorsum or dorsal margin, and the concave 

 side is called the venter or ventral side. 



The valve is cymbijorm when it has margins unequally curved in 

 opposite directions as in many Cymbella. 



In this case we have similarly applied the term dorsal margin to the 

 most convex side, and ventral side or margin to that which is less 

 convex and situate on the concave side of the raphe. 



The valve is bent when the sides are curved alike and in the same 

 direction and geniculate or genuflexed when either one or sometimes 

 both sides have an acute fold. 



2ND. Outline. The forms of the valve may be classified into two 

 groups ; firstly, forms which are circular or derived from a curved 

 line, and secondly, those which are polygonal. 



