324 DESCRIPTION OF [Poli/(/astrica. 



diameter of the bases ; hence the cell appears almost square (P. 24, 

 f. 28) on the broad side, on the narrow side a longish rectangle. On 

 the bases are four ribs, parallel with the broad diameter of the ellipse 

 The bases also present many slight indentations. The ribs are merely 

 in the membrane, as may be ascertained by a side view. They 

 extended, however, a short distance on both sides, over the lateral 

 surface of the cylinder, and are gradually lost. The nature of the 

 ribs is not to be made out in these plants, from analogy to the 

 above-described (see genus GaUioneUa nov. sp.) It becomes probable 

 that they are formed by secreted extra cellular substance. Sec 

 description of Plate 24, figs. 26, 27, and 28. 



The cell-contents consists of a colourless transparent fluid, and a 

 free nucleus-like heap of chlorophyl, mingled with mucilage In 

 propagation, this green mass divides into two parts, between 

 which the septa appears. 



: Among the Bacillaria figured by Ehrenberg, it exhibits the 

 greatest affinity to B. cwieata, and is almost of the same size, viz- — 

 l-1152nd to l-1200th. The latter, however, is distinguished. — 1. 

 By the alternating broader lateral borders. 2. By the individual 

 cells remaining attached together at one angle after division. 3. By 

 the strife not being straight upon the dividing wall, and not parallel 

 with the axis, but running at an oblique angle over the broad lateral 

 surface ; and 4. By the additional existence, on both sides, of two 

 striae on the lateral margin (on the narrow lateral surface.) 



Unlike the ordiuaiy members of the genus Bacillaria, the indivi- 

 duals of the species in question only remain united together imme- 

 diately after the division of the parent cell, subsequently they are 

 completely separated, or, in other words, are free. (Bay Society, 

 1844, p. 267.) 



Some of the preceding species of Bacillaria will be found again 

 referred to, with additional matter, under the genus Biatoma. 



Genus Biblaihtjm. — Lorica tabular, made up of many lamina ; 

 transverse in relation to the length ; central portion smooth, but the 

 lateral segments, like the covers of a book, ornamented with trans- 

 verse striae. The smooth central portion of each individual, is 

 furnished with a large umbilicus at its middle. There are quadi'an- 

 guLir, elliptical, and cylindrical varieties in this genus. 



