PLEUROSIGMA. 25 1 



/. Strice decussate (cutting one another in three directions). 



A. All striae almost equally delicate. 



* OBLIQUE STRLE CROSSING ONE ANOTHER IN THE MIDDLE OF THE 



VALVE AT AN ANGLE OF ABOUT 6o. 



PI. angulatum W. Sm. (S.B.D., i., p. 65, pi. 21, f. 205; 



H.V.H. Atl., pi. 18, f. 2, 3, 4* ; Type No. 169), plate 6, fig. 257. 



Valve broadly lanceolate, gently flexed, sigmoid, with median part slightly 

 angular. Raphe feebly sigmoid. Striae decussate, having the same direction 

 throughout the surface of the valve, 18 to 20 in 1 c.d.m., the transverse striae 

 slightly more approximate than the others. Length, about 15 c.d.m. 



Marine. Common. Antwerp (Scheldt), Blankenberghe, Ostend ; Norfolk (Kitton). This 

 species and its varieties are found on all the coasts of the North Sea. 



var. ^stuarii. {PL Mstuarii IV. Sm. /; H.V.H. Atl., pi. 18, f. 8* ; 

 Type No. 172), plate 6, fig. 258. 



Differs from the preceding, with which it is often mixed, by its rather 

 smaller size, and its slightly rostrate-produced apices. 



var. quadratum. {PL quadratum IV. Sm. / ; H.V.H. Atl., pi. 18, 

 f. 1*; Type No. 171), plate 6, fig. 259. 



Differs from the type form by its larger size, and its more quadrangular 

 shape. 



var. major. (H.V.H. Atl., pi. 18, f. 5*), plate 6, fig. 260. 



Differs from the type form by its much larger size, reaching 22 c.d.m. 



Blankenberghe. 



var. delicatulum. {PI. delicatiilum IV. Sm. ! ; in H.V.H.'s Types 

 No. 74 and 172). 



Valve very narrow and infinitesimally attenuate. Striae 24 or 25 in 

 1 c.d.m. Length, 16 to 18 c.d.m. Breadth, about 2 c.d.m. 



Blankenberghe. Rare. 



var. strigosum. {PL strigosum W. Sm. /; H.V.H. Atl., pi. 19, f. 2* ; 

 Type No. 170), plate 6, fig. 261. 



Of very considerable size (more than 30 c.d.m.) with margins 

 infinitesimally sigmoid, not angular, with apices sub-obtuse. 



This form has not been found in Belgium. Norfolk (Kitton), Coast of Sussex (W. 

 Sm.), Cheshire (Comber), Humber (Norman), Ireland (O'Meara). 



PI. rigidum W. Sm. (S.B.D., i., p. 64, pi. 20, f. 198 ; H.V.H. 



Atl, pi. 19, f. 3* ; Type No. 178), plate 6, fig. 265. 



Valve almost straight, with apices obtuse, sometimes slightly inflated. 

 Raphe robust, straight, with apices just a little flexed, often with a lateral row 



