402 DESCEIPTION OF \_Poli/<jastrica. 



to air broatliing plants, and absent in submerged ones. In S. bine- 

 riata, and S. splcnd'da, the costse or undulations caused by these tubes 

 are continued to the margins of the alse, and gives a singularly beau- 

 tiful appearance to the front view of the frustule." 



The so-called costae are otherwise called, by Ehrenbcrg, transverse 

 Btriaj or pinnules ; but in the species named, as well as in others, the 

 strice are not mere single lines, but have a double contour, and an 

 apparent prominence or depression. 



SuKiiiELLA Lihrile. — Elongate, oblong, slightly constricted at the 

 middle ; ends sub-acute, rounded or apiculate ; the last condition is, 

 however, more common with young or smaller specimens. Transverse 

 Btria3 eight in 1-1 200th. Lateral aspect linear, oblong, with rounded 

 truncate ends, divided by a central clear line, on each side of which 

 is a wavy band having about six undulations. Group 155, repre- 

 sents both aspects. Alive at Gravescnd and elsewhere ; fossil at San 

 Fiorc, Mexico, &c. 



S. striatula. — Ovate dorsally, with small aloe ; striae strong, curved ; 

 ends rounded (fig. 137); laterally (on front view. Smith) cuneate 

 and elliptical ; ends rounded, very broad, (fig. 138); alae small; striae 

 eight to thirteen in 1-1 200th. This form was discovered by Dr. 

 Surircy in 1826, and preserved alive foj' eighteen months. It is veiy 

 transparent and colourless ; its motion, when observable, is slow. 

 Alive on the English and French coasts; fossil in Bohemia. Length 

 1 -3450th. to l-60th. 



S. tmdulata=^Denticida constricta (Klitz.) — Striated, large, elliptical, 

 ends rounded; laterally, linear and truncate; margin prominently den- 

 tate, with a ficxuosG baud running longitudinally on each side. 

 Striae four in l-1200th. (fig. 149 represents an oblique view.) Found 

 amongst Oscillatoria. Length 1-2 10th. 



S. constricta = Denticula undulata (Kiitz.) — Striated, large, oblong, 

 sliglitly constiT-cted at the middle on the ventral surface ; ends obtuse, 

 truncate; striae three to four in 1- 1200th. Laterally oblong, extre- 

 mities dilated, rounded; the margin dentate. Alive at Berlin. 

 Length 1-2 10th. 



S. constricta (Smith) — Frustules on front view oblong, with 

 rounded ends ; outline on side view elliptic-lanceolate, each margin 

 having a central sinus ; alaj distinct ; costic numerous, delicate ; 



