CLASSIFICATION 201 



siderably longer than the diameter of the valve. Frustule weakly siliceous, extreme de- 

 velopment of the connecting zone. 



Araphidineae. Valves linear to linear-lanceolate, straight or arcuate, sometimes 

 spathulate, nodules absent. Pseudoraphe present or absent. Mainly colonial forms. 



Raphidioidineae. Valves arcuate, sublunate to hemispherical, or club-shaped. 

 Rudimentary raphe on one or both valves, developed in the polar areas; no central 

 nodule. 



Monoraphidineae. Valves oval, oval-lanceolate. Valves dissimilar, one bearing a 

 true raphe, the other a pseudoraphe. 



Biraphidineae. Valves lanceolate to oval, sometimes sigmoid, arcuate or sublunate, 

 showing isobilateral, zygomorphic or dorsiventral symmetry. Flat or twisted. Polar 

 and central nodules usually well developed. A true raphe upon each valve. 



Surirellineae. Valves oval-lanceolate, obovate or subrectangular, flat, twisted or 

 genuflexed, nodules absent. Each valve furnished with a pseudoraphe in the polar axis 

 and peripheral alate canal-raphe. Frustules solitary. 



The first five suborders correspond to Schiitt's Centricae, but I find great difficulty in 

 referring the structure of many of the Auliscineae and Soleniineae to a radial or con- 

 centric framework, and find it quite impossible in the Biddulphiineae and Anaulineae. 



The Biddulphiineae are easily understood if the structure of Triceratium fovus is con- 

 sidered. In this biddulphioid diatom there exists a peculiar form of polymorphism (the 

 word is used in its widest sense) which provides a series of forms possessing from two to 

 thirteen angles. The structure of the valve may be likened to the effect produced when 

 a sheet is flung upon a number of short stakes erected at equal distances from each other 

 and in some definite geometrical pattern. The sheet may be said to be slung " hammock 

 fashion" upon the stakes. In the same way the valve substance of the biddulphioid 

 diatoms must be considered as being stretched upon a number of circumferential foci. 

 The cornutate processes of the valve are the main features of the valve view, and the 

 structure of the valve is usually, arranged with reference to the adjacent process, each 

 angle with its reinforced horn providing an independent growing point. Young or 

 immature valves formed within the parent frustule during multiplication by fission are 

 frequently found to produce the processes first. 



This method of growth, which involves the establishment of peripheral "growing 

 points" parallel to the principal axis of the frustule, is quite different from the type of 

 development found in the Discineae. A similar type of peripheral structure, which 

 adopts bipolarity as an outstanding feature, is seen in Chaetoceros, Hemianlus, Attheya, 

 Eucampia and many other genera. 



The last five suborders correspond to Schiitt's Pennatae, and in the main they agree 

 with the idea of isobilateral construction. Difficulties in this respect are, however, en- 

 countered in the genus Campylodiscus. 



Campylodiscas and Surirella have been separated from the Biraphidineae on account 

 of the complex raphe system of the former. The Surirellaceae have a different valve 

 structure from any of the families of the Biraphidineae. On both sides of each valve of 



