43O PHYSICAL GEOGRAPHY. 



are either short and stout or long and slender, varying in this respect in 

 different species. They also vary in number from one upwards, the 

 number being constant in some species, while in others it varies very 

 greatly. Usually the outermost end is somewhat swelled into something 

 like a knob, and within this part the central canal, running through the 

 foot, also swells into a spherical cavity. When a perfect specimen of an 

 Aulacodiscus is examined on what is known as its "front view," that is to 

 say, with the edge of the valve presented towards the eye, the presence 

 of the feet makes the appearance of this diatom very characteristic. The 

 description of this genus has been thus full, because the beautiful com- 

 plexity of the diatomaceae could thus be made evident, and many points 

 of structure dwelt upon at the outset. So when we come to describe a 

 few of the other forms as types, their resemblance to or variance from 

 Aulacodiscus will be noticed. 



The genus Coscinodiscus has the same general characters of outline 

 and sculpture of markings as the genus just described, but is destitute of 

 "feet," and, therefore, of the raised portion upon which they are placed. 

 The two membranes are present, but the inner one is smooth. Both 

 Aulacodiscus and Coscinodiscus are inhabitants of salt water, although 

 there is a minute form, usually classed as a Coscinodiscus and called 

 minor, which has been seen in fresh water ; but it is now pretty well 

 ascertained that it is not a true Coscinodiscus, but belongs to the genus 

 to be described next. 



In Melosira, the frustules have a general resemblance to Coscinodiscus, 

 and are frequently mistaken for specimens of that genus, especially when 

 dead and detached one from the other. Usually, however, the valves are 

 so much more convex when viewed edgewise, that the whole frustule 

 may approach in form a sphere, as is the case in the species chosen as 

 an illustration. It however differs widely from Coscinodiscus in having 

 its frustules united, by means of their valves, into long chains, which are 

 quite flexible, so that they wave about in the moving water. Some 

 species have the valves more flat, and then the live plant looks like a 

 number of pill-boxes attached together. Some species are peculiar to 

 fresh water, whilst others are found in the sea; but it would seem that a 

 few of them can become acclimated to and live in either kind of water. 



ActinoptycJius, another beautiful genus, contains several species which 



