206 DESCRIPTION OF [Polygastrica, 



The siliceous external coat is often very beautifully sculptured; 

 the markings being modelled on the living organic membrane. Much 

 discussion has arisen as to the true nature of these markings, which 

 assume the appearance of dots fpunctaj, ribs fcostce), stria?, pinnules 

 (pinnae), furrows, or lines — whether the dots are actually pores, as 

 generally surmised by Ehrenberg, and whether the ribs, stria3, or 

 lines, are depressions or elevations of the surface, or even sometimes 

 figures. The opinion seems to be gaining ground, that the lorica 

 is entire without pores or fissures ; in favour of this opinion, are 

 Balfs, Nageli, Dujardin, the Eev. W. Smith, and Schleiden. On the 

 contrary, Klitzing supposes the gelatinous investment of many 

 Diatomece to issue from pores in the lorica ; and Ehrenberg believes 

 in the existence of both pores and fissures, supposing the former to 

 give exit to organs of locomotion and the latter to ova, &c. (See 

 genus Navicula, and plates 19 and 20.) In several genera, for 

 example, SJiqjodiscus, Cemtaulus, Rhizoselenia, &c., the lorica produces 

 more or less extended processes, simple or branched (ramose), which 

 are generally known as cornua, sometimes as tuhuU (Ehr.) It often 

 happens that the apparent lines, or bands, on the surface, are merely 

 indications of internal partitions (septa), and which are either com- 

 plete or perfect (/. e., entirely dividing the internal cavity of the 

 lorica), or incomplete (imperfect.) To the partial septa, as ia Gram- 

 matophora, Kiitzing has assigned the term viU(2. " The lines," 

 says Niigeh, speaking of a form allied to GalUonella (Elu-.) "which 

 would intimate a division of the shield into two or more pieces, are 

 the septa by wliich the cell-division is effected." 



The puncta, costa), and other external markings, as also the 

 processes of the lorica, are valuable in forming specific and generic 

 characters. The presence or absence of a ccnti'al pore often forms a 

 leading characteristic. Kiitzing, indeed, rests his great division of 

 the BiatomecB into StomaticcB and AsfomaticcB, on this peculiarity. 



The distinctness of the various markings of the lorica is much 

 interfered with by the presence of the lining membrane, which should 

 therefore be destroyed by maceration in nitric acid, or by exposui-e to a 

 strong heat. 



The cavity of the lorica, in the true Biatomacece, is now generally 

 admitted to be single ; in other words, their frustules are uni-cellular. 



