332 DESCRIPTION OF [Pul^ffagtriea. 



middle of each piece, but without terminal pores. Never developed 

 in the form of a chain or cluster ; propagation by self- division, or 

 gemmules, is doubtfnl ; mostly striated ; the transverse striae appear 

 to be internal flutings like ribs. The lorica is composed of two lateral 

 pieces, joined together at a central fiirrow, somewhat resembling the 

 keel of a boat, the under surface being flat, the upper somewhat 

 arched. A foot-like process has been seen projecting out of the 

 central opening, on the under surface. The internal matter is green 

 or yellow, and often appears to be fonned in two plate-like masses. 

 Though actual motion has not been observed, change of place appears 

 to ensue. 



Kiitzing makes Cocconeis the type of a family Cocconeidm, order 

 Stomatic(B, (see page 308.) In defining the genus Cocconeis he is at 

 variance with Ehrenberg, in stating that its members are ultimately 

 adnata (attached) and sessile. 



Cocconeis bears the greatest resemblance to Exinotia (Ehr.) and 

 Upitliemia, (Kiitz.) but is at once distinguished from them by 

 having a central aperture, whilst it wants terminal openings. The 

 central umbilicus is found only on one siu'face ; the idea of Ehrenberg, 

 that a foot-like process can be protruded from this spot, is uncon- 

 firmed; indeed, it is pretty weir^fecertained, that the supposed openings 

 are not actually such, but either depressions or elevations, more 

 probably the former, of the surface of the lorica. 



The development of Cocconeis by self-fission, considered doubtful 

 by Ehrenberg, is spoken of as unquestionable by Kiitzing, who also 

 describes the very smaR specimens as occurring together in great 

 number, and enveloped in a gelatinous substance. Moreover, little 

 doubt remains, from the researches of Mr. Thwaites and others, that 

 Cocconeis, like its allied genera Cocconema and Eunotia, is propagated 

 by the process of conjugation. The members of this genus are met 

 with abundantly, both in fresh and salt water, floating about in an 

 isolated manner, or more commonly still, parasitic on Algse, such as 

 Cludoplwra, Callithamnium, = Polysiplionia, Conferva, &c. They 

 occur also in the fossil state. The marine species are most beautifully 

 sculptured, whilst the fresh water are smooth. 



Cocconeis scutellwn. — Elliptic, convex on one sui'faccj with trans- 



