248 



DONKINIA. 



GENUS 21. DONKINIA RALFS, i860. 



Valve carinated, with keel sigmoid, inter- 

 rupted at the median portion by the central 

 nodule. Keel not accompanied laterally by 

 projecting lines (or alae). Frustule strongly 

 constricted at the median portion. 



ANALYSIS OF SPECIES. 

 Fig. 49. Donkinia recta. 



r Strise rectangular, the transverse 20, the longitudinal 21 in 1 c.d.m. 

 \ Striae decussate, fine, 21-24 m l c.d.m. 



D. recta. 

 D. carinata. 



D. recta (Donkin) Grun. (Pleurosigma rectum Donk., T.M.S., 



1858, vi., p. 23, pi. 3, f. 6 ; H.V.H. Ad., pi. 17, f. 9*; Type No. 194), plate 

 5, fig. 286. 



Valve broadly linear, with apices attenuate, subcuneiform-acute. Raphe 

 strongly sigmoid. Strise reaching to the raphe, rectangular, about 2 1 in 1 

 c.d.m. Girdle face strongly constricted at median portion. Length, about 

 8 or 9 c.d.m. 



Marine. Washings of mussels. England, Ireland (O'Meara). Rather common on all the 

 coasts of the North Sea. 



var. angusta H.P. {Pleurosigma angustum Donk.). Narrower, with 

 keel more eccentric. 



England, Ireland (O'Meara). 



var. minuta H.P. (Pleurosigma minuta Donk.). Small form of 

 type. Length, about 6 c.d.m. 

 Rather common. Ireland (O'Meara). 



D. carinata (Donk.) Ralfs (Ralfs in Prit. Inf., p. 921, pi. 8, 

 f. 49*), plate 35, fig. 912. 



Valve straight, linear-lanceolate, with apices acute, very convex. Raphe 

 very strongly sigmoid. Striae decussate, fine, 21 to 24 in 1 c.d.m, Length, 

 12 c.d.m. 

 Marine, England, Ireland (O'Meara). 



