lwa-xaiu.j lOI 



The length of such a common specimen as Navicula lyra is about 

 i-25oth inch, and the breadth about i-3rd of that size, (d) — Their 

 formation and growth. Each individual diatom, called a frustule, 

 consists of a single membranous cell, enclosing, in addition to the 

 liquid of the cell, a nucleus, surrounded by protoplasm, some oil 

 globules, and a brownish matter called endochrome, which is 

 composed of chlorophyll and diatomin. This cell is enclosed 

 in a silicious covering, generally forming a kind of box, consisting 

 of two valves and a zone or connecting band. This characteristic 

 provides the name for the group, from the ease with which 

 the parts separate from each other. In a large proportion of 

 diatoms the frustules are met with entirely free — viz., Triceraiium, 

 Pleurosigma, Snrirella, Coscinodisats, &c. In others they develop 

 in zig-zag chains, as Isthmia, Grammatophora, Diatoma vulgare, 

 &c. In others, again, they develop in long filaments, as Melosira, 

 Achranihes, &c; in curved filaments, like Meridion ciradare ; and 

 in beautiful fan-shaped growths like Liemophora flabellata. Other 

 varieties, again, grow by a stipe or stalk-like appendage, as in 

 Gomphonema and Mastogloia, while some have a mucous or 

 gelatinous investment, as in Schizonema grevillii. (e) — Their 

 spontaneous movement. This has been the subject of much 

 discussion, and has been explained in many ways. Probably 

 the most likely cause is the action of changes resulting from 

 the nutrition of the cell. (/) — Their collection and examination. 

 On this part of the subject Mr. Maxwell gave many practical hints 

 and brought for the inspection of the members his own instruments 

 and a very varied collection of mounted and unmounted speci- 

 mens. For the examination of diatoms the very finest objectives 

 are employed, but much practical and useful work may be 

 accomplished by moderate equipment. Many of the members 

 had an opportunity of examining the very fine markings which 

 have rendered the diatomacece so famous, and which have 

 been a very important indirect cause of the modern advance in 

 the manufacture of high-class objectives. (g) — Their practical 



