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pose ; that of exhibiting characters peculiar to genera and spe- 

 cies. Of course we must leave entirely out of the question all 

 such objects as muds, guanos, dredgings, &c., which can very 

 rarely, if ever, be used for the purpose of exhibiting species. 

 Gatherings containing many species in a mixed condition should 

 invariably be rejected, unless they contain something of special 

 importance, such as rare species, or some large and fine speci- 

 mens of common species. In this paper it is my intention to give 

 such plain and concise directions as will, I hope, materially aid 

 students in mounting these interesting organisms in such a way 

 as to exhibit their characteristics in a proper manner. For many 

 of the hints I am about to give I am indebted to Professor Walker- 

 Arnott of Glasgow, Scotland ; the rest are the results of the ex- 

 perience of about six years spent in this pursuit. For the method 

 of cleaning guanos, infusorial earths, muds, &c., the reader is 

 referred to a paper, by the present writer, published in the sev- 

 enth volume of the London Journal of Microscopical Science. 



All gatherings should be made in as clean a state as possible in 

 the first place, as it will be found difficult to clean them after- 

 ward. The Diatomaceae are to be found growing in both fresh 

 and salt water, either attached to submerged aquatic plants of 

 laro-er growth, or floating freely on the surface. It is a mistake, 

 that most writers on this subject have fallen into, to term any of 

 the Diatomaceae parasitic, as they do not subsist on nutriment 

 derived from the plant, or other substance, to which they are 

 attached. The attached species might more properly be termed 

 epiphytaceous, and the free ones eleutheraceous ; many species, 

 however, there is little doubt, ar^ at one period of their existence 

 fixed, and at another free. That there are any true free spe- 

 cies is unestablished ; that is to say, I very strongly suspect 

 that all of our so-called free species are fixed during the earlier 

 period of their growth. Some species are to be looked for on the 

 surface of the muddy bottom, or in the stomachs of certain marine 

 creatures, as the Salpce, the Noctiluca miliaris, &c. A peculiar 

 method of procuring these last is used, and wall be described. 



Diatomaceae are to be looked for in almost every stationary 

 piece of water, and, in some cases, in transient pools, the result of 

 flooding or rain. Freshwater species will be found attached to, 

 or entangled among, the leaves and stalks of larger submerged 



