86 



TERMINOLOGY AND CLASSIFICATION OF DIATOMS. 



A. Valves, whose outlines are derived from a curved line, may be : 

 Circular or orbicular : i.e., forming a circle. Ex. Coscinoaiscus 



(pi. 23, f. 664-668). 

 Hounded: i.e., of an approximately circular form. Ex. Surirella 



cruniena (pi. 13, f. 586). 

 Oval: i.e., in the shape of an egg. Ex. Surirella striatula (pi. 13, f. 580). 

 Elliptic : i.e., the outline forms an ellipse, that is to say, an 



elongated circle whose two extremities are equally rounded. 



Ex. Cymatopleura elliptica, type form (pi. 12, f. 480b.) 

 Reniform : i.e., in the shape of a kidney. Ex. Surirella reniformis. 

 Lanceolate : i.e., in the lorm of a lance head with narrow and 



somewhat pointed apices. 

 Cuneate or cuneiform : i.e., in form of a wedge with rounded 



angles, as in many Gomphonema (pi. 7, f. 301, 302). 



B. Valves whose outline assume a polygonal form, may be : 

 Triangular : i.e., having three angles as in the Biddulphia 



{Triceratium) Favus, type form (pi. 21, f. 643). 

 Quadrangular (pi. 21, f. 642), and pentangular as in certain Triceratium. 

 3. Extremities (Apices). The apices of valves exhibit characteristics 

 of great importance for descriptive purposes. They may be : 

 Acute or sharp-pointed : i.e., they imperceptibly taper to a point. 



Obtuse : i.e., they 

 are more or less 

 blunt at the point. 



Truncate : i.e., 

 they appear to be 

 cut off abruptly. 



Attenuate and 

 tapering, when they 

 lessen in breadth 

 abruptly, a modifica- 

 tion which authors often improperly designate by the term produced. 



Acuminate : i.e., they terminate more or less abruptly at the apex in a kind 

 of narrow extension. Two kinds of acuminate termination can be distinguished. 

 1 st. The acuminate termination properly so-called, that is to say, the 

 narrowing of the conical point (fig. 23, A.B.). 



2nd. The rostrate, when the narrowing takes the form of a beak with 

 parallel margins (fig. 23, C), as in Navicula Amphirhynchus (pi. 5, f. 214). 



When, after the narrowing, the valve is again enlarged in the form 

 of a head, it is called capitate. The valve may therefore be acuminate- 



YY^vy 



ABC D E 



Fig. 23 Different forms of acuminate termination 



A.B. Acuminate apices. 



C. Rostrate apex. 



D. Acuminate-capitate apex. 



E. Rostrate-capitate apex. 



