

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 /x. 



Sub-order 5. SURIRELLOIDEJE. 



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. SURIRELLACEJE. 



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 

 rarelv 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 r////^//^'''"'"- 



B. Valve-faces not undulate. 



* Valves elliptical or keel-shaped, straight, 

 with strong transverse ril>s; 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 

 ( if other- Ca mpylodiscus. 



