XVI INTRODUCTION. 



some species these interruptions are more numerous, being found 

 along the entire line of suture, and are often connected with 

 minute canals, hollowed out between the siliceous epiderm and 

 internal cell-membrane, and apparently formed by waved flexures 

 of the epidermal envelope. These are noticed under the genus 

 JEpithemia, p. 11. They are very conspicuous in Epithemia 

 longicornis, Supp. Plate XXX. fig. 247, and form distinctive cha- 

 racters in the genera Si/rireUa and Campi/Iodiscus. I have em- 

 ployed the term " Toramina," in reference to these perforations 

 in the siliceous epiderm, and the epithet " Canaliculi," to de- 

 signate the grooves into which they occasionally open. 



The connecting membrane, being of later formation, is usually 

 less firmly sihceous than the epidermal valves, and rarely pre- 

 sents much complexity of structure. Not so with the valves 

 themselves ; we have here the greatest variety of form and 

 diversity of structural arrangement. 



These variations in form and structure will be best understood 

 by an inspection of the figures, and wiU be noticed more par- 

 ticidarly under each genus. I shaU confine my remarks at pre- 

 sent to a few general characters. 



In all, we find the valves presenting a surface of a greater or 

 less degree of convexity. This follows as a necessary conse- 

 quence, from their enclosing, when applied to each other, the 

 space occupied by the internal cell. This convexity is often 

 that of a regularly cm'ved siu-face, forming a segment of a circle 

 of a greater or less radius. The segment is occasionally that of 

 a perfect hemisphere, and the valves, when applied to each 

 other, form a miniatm-e globe ; but more frequently the cm'va- 

 ture is slight, and extends over a part of the valve only, which 

 then appears as a portion of a hollow tube, closed at one end, 

 and which may be either cylindrical or compressed, triangular 

 or quadi'ilateral. 



The intimate structm'e of the valve presents an amazing 

 variety, in every genus, and almost in every species, offering 

 distinct features, which serve as the best and most facile means 

 of distinction and identification. Strise or lines frequently mo- 

 niliform, dots arranged in a radiate or concentric manner, and 

 minute divisions presenting perfectly hexagonal outlines, are 



