Phi/ip : The Diatoms of the Sedbergh District. i^C) 



of the DiatomacecT nearest in apparent structure to the 

 Radiolaria are the earhest to appear in fossil deposits. These 

 all belong to the sub-family, Ana-Raphidieae, and are circular, 

 sub-circular, triangular or polygonal in outline, and frequently' 

 furnished with processes, spines, or awns. There is a fossil 

 deposit found at Oamaru in New Zealand, in which a consider- 

 able number of very beautiful species well known in the " show " 

 slides, issued by those who mount for sale, are found. These are 

 without exception, all belonging to the Ana-Raphidieae. It is 

 clearly a marine deposit from a warm sea rich in silicon, and 

 besides the diatoms, contains an abundance of radiolaria and 

 sponges, which also require this element to build up their skele- 

 tons. A somewhat later deposit of a similar character is that 

 which underlies the city of Richmond, Virginia, but in this, 

 while the great majority of species are those of Ana-Raphidieae,. 

 there is also a small but significant number of Raphidieae. 

 The collections made by Cleve and others from the Plankton 

 of various seas to-day, consist also in the main of Ana-Raj)hi- 

 dieae. Hence we may picture the earliest diatoms as belonging 

 to this sub-family, such forms as the first three figures in our 

 plate, floating on the surface of the sea, moved only by the 

 winds and waves, having no power nor need of self-motion. 

 Next we must conceive that quantities of these forms would 

 be carried by currents to the margin of the sea, and from being 

 jnirely oceanic, would be compelled to adjust themselves to a 

 neritic existence covering the surface of rocks and mud on the 

 shore. Here the continual deposition of silt washed by the 

 waves from clay cliffs, or brought down by rivers would be the 

 greatest danger against which the organism would have to 

 struggle, and forms unable to lift themselves to the surface of 

 the mud, would be buried and stifled. Hence we have the 

 development of the Raphidieae, with their bilateral instead of 

 centric shape, their raphe, and its associated power of self 

 motion. Most persons, when they first see through the micro- 

 scope living naviculas passing hither and thither across the 

 field of view, are apt to think this motion is at least as free and 

 voluntary as that of the Infusoria. Further investigation, 

 however, reveals that it is strictly limited to an alternating 

 movement, a certain distance in one direction, a pause and a 

 return. How effective this motion is, nevertheless, for the 

 purpose for which it was evolved may be seen any day bj^ col- 

 lecting a gathering of Pleurosigmas from the mud of our tidal 



1910 Apl. I. 



