Surirellacece 303 



are displaced to the same side of the frustule. The valve-view is 

 somewhat curved with rostrate apices, and the girdle-view is 

 straight and linear-rectangular. The carinal dots are very con- 

 spicuous. 



H. Amphioxys (Ehrenb.) Grun. is the only freshwater species and is a 

 frequent British Diatom. It often occurs in prodigious quantity on damp 

 earth ; length of valves 45 75 p. 



Sub-order 5. SURIRELLOIDE^. 



The frustules are generally symmetrical with regard to the 

 sagittal axis and each valve frequently possesses two rows of wing- 

 like projections (or alse). There is a median pseudo-raphe without 

 any trace of nodules, which is sometimes situated on a sagittal 

 keel. The valves are generally strongly costate. 



Family 1. SURIRELLACE^3. 



The Diatoms of this family exhibit considerable variety of 

 form, although they are almost always symmetrical with regard to 

 the sagittal axis. In the valve-view they may be elliptical, linear, 

 ovate, cuneate, or subcircular, with a median pseudo-raphe from 

 which radiate strong costse (or ribs). In some forms the valves 

 are winged, each being furnished with wing-like outgrowths which 

 project outwards at the junction of the valve and the girdle. The 

 pseudo-raphe is sometimes situated on a pronounced sagittal keel. 

 The valve-faces are occasionally undulate, the undulations showing 

 plainly on each side of the girdle-view, and the frustules are 

 rarely twisted. There are two large chromatophores in each 

 cell, one within each valve-face, and sometimes with projecting 

 internal lobes. In all cases the frustules are solitary and free- 

 floating. One auxospore is produced by the conjugation of two 

 mother-cells. 



The family includes the three following genera : 



A. Valve-faces undulate Cymatopleura. 



B. Valve-faces not undulate. 



* Valves elliptical or keel-shaped, straight, 

 with strong transverse ribs ; pseudo-raphe 



of one valve parallel to that of other Surirella. 



** Valves circular and saddle-shaped ; pseudo- 

 raphe of one valve at right angles to that 

 ofother Campylodiscus. 



