374 SURIRELLA. 



II. Valves twisted, raphes of the two valves parallel. 



S. spiralis Klltz. (Baa, pi. 3, f. 64 ; Campylodiscus spiralis 

 IV. Sm. ; S.B.D., i., p. 29, pi. 7, f. 54; S. flexuosa E/ir., Amer., 1843. p. 

 136, pi. J. 3, f. 1-20; S. torta Breb. ; H.V.H. Atl., pi. 74, f. 4-7 * ; Type No. 

 434), plate 13, fig. 592. 



Valve elliptic lanceolate, twisted round the longitudinal axis ; costa; 

 robust, 2 to 3 in 1 c.d.m., reaching close to the median line ; striae fine, but 

 well marked, about 26 to 28 in 1 c.d.m.; here and there throughout the 

 length of the costre some coarse scattered puncta. Frustule twisted 

 into a figure of 8, with connecting zone rather broad, showing the aire 

 distinctly, with raphes parallel and superposed on the two valves. Length, 10 

 to 13 c.d.m. 



Freshwater. Rather rare: Dieghem (Del.), Antwerp. England (W. Sm., Kitton in Baxter 

 Coll., 3507), Ireland (O'Meara), France (Brb.), etc. 



By the side of Surirella are placed the two following genera, which have 

 not been generally adopted. 



Stenopterobia De Breb. in litteris. Frustules very elongated and 

 very narrow, sometimes sigmoid. 



In this are comprised the Surirella Baileyi Lew., S. intermedia Lew., S. 

 delicatissima Lew., and S. anceps Lew., which appears to be a Synedra furnished 

 with coarse marginal sparse puncta. 



These four forms are American, and are figured by Lewis in " On some 

 new and singular intermediate forms of Diatomacea." 



The S. anceps Breb. is found in Type No. 291, which is said to have come 

 from Cornouailles. 



Plagiodiscus Grun. and Eul., 1868. Valves reniform with 

 radiant costas, includes two forms, Plagiodiscus nervatus Grun., and PL 

 Martensianus Grun. These are abnormal forms of a Surirella, closely allied 

 to, if not identical with, S. Gemma. 



