426 DESCEiPTiON 01? {^Fvlygastvica. 



Statjeopteea microstcmron. — Styliform and linear on ventral surface ; 

 sides straight ; suddenly and sharply constricted, with wide, ohtusc, 

 rounded apices. Brazil, Labrador. 



S. parva. — Small, styliform, and linear ; on rentral surface witli 

 marginal lines, sides not quite straight ; ends rather constricted and 

 widely ronnded. Mexico. 



S. scalaris. — ^Very small ; bacillar on nnder surface ; straight, viewed 

 laterally; ends rounded, pinnules strong, and twelve in 1-1 200th. 

 Labrador. (P. 15, f. 10, 14, and 30.) 



S. dendrolafes. — Yentrally, naiTowly linear ; ends obtuse ; border 

 finely and obliquely striated ; on lateral aspect broadly oblong, 

 quadrate. Length 1 -490th. Has been seen divide longitudinally. 



Sub-genus Pleukosigma (Smith.) — Yalves convex, sigmoid, striated ; 

 strise resolvable into dots. 



This genus, so characterised, comprises several members of Ehi'cn- 

 berg's sub-genera Nmicula and Ccratoneis, and would probably 

 embrace, also, some of the 8tauropiera. 



The shells of the different species of this sub-genus, are much 

 employed as tests for microscopes. To exhibit the striae, requires 

 oblique ilhimination, and an object-glass of large angular aperture. 



* Beads alternate, striae oblique ; all marine. 



P. formosum. — Yalve linear lanceolate, gradually attenuated to 

 the somewhat obtuse ends, tTvisted ; median line broad, not central ; 

 colour bright chestnut-brown. Length l-66th. Greatest breadth 

 1 -850th. Oblique strise 1 -3000th inch apart. Shoreham Harbour. 

 Tlais species is well distinguished by the position of its median line, 

 which, owing to a twist in the valves, appears nearly to coincide with 

 the edges for a considerable distance at either end, and then crosses 

 in a diagonal direction (P. 19, f. 1 and 2 ; the latter a portion magni- 

 fied 5500 diameters.) 



P. speciosum. — Yalve linear, slightly attenuated, somewhat twisted, 

 extremities obtuse, median line not ccnti'al ; colour a pale straw. 

 Length l-85th. Greatest breadth 1 -850th. Stria) l-40,000th apart. 

 It is straighter, stouter, and more delicately marked than the 

 former. (P. 19, f. 3.) 



P. elongatum. — Yalve linear laucculate ; flexure slight, extremities 



