PROCEEDINGS OF SOCIETIES. 115 



the minute claw-like appendage at the apex of each frustule is 

 inserted in a corresponding depression on the bevelled surface of 

 the frustule with which it was in apposition, as if with the view 

 to give additional support at the point of union of adjacent 

 frustules. 



From the profusion in which these flocculent masses of Ehizo- 

 selenia occur, and their rapid accumulation to a greater and 

 greater extent so long as calms prevailed, it seems probable that 

 at some depth below the surface they may form considerable 

 layers ; and this view is further borne out by the fact that the 

 digestive cavities of Salpae and certain other oceanic Hydrozoa 

 are at times found almost entirely filled with the frustules. On 

 the Atlantic side of Africa Dr. Wallich captured salpse in chains, 

 numbering from half a dozen to a score individuals, each five or 

 six inches in length, the digestive sacs of which, measuring nearly 

 three quarters of an inch in diameter, were completely distended 

 with this organism only. 



Dr. Wallich wished to draw attention to this fact for another 

 reason, namely, that it would indicate the possession by these 

 humbly-organized beings of a power to search for and pick out 

 from amongst a variety of free floating microscopic algae a pai'- 

 ticular form ; unless it be assumed (which is far from probable) 

 that, having incepted a single frustule, this retains the faculty of 

 growth and multiplication within the cavity in which it becomes 

 imprisoned. 



Dr. Wallich invited the attention of those who have oppor- 

 tunities of carrying on microscopic investigations at sea to the 

 influences (whatever they may be) which cause the minute algse 

 of the open ocean to rise at certain periods to the surface, and 

 again to descend to unknown depths. He suggested that atmo- 

 spheric pressure, or the more ready transmission of light and heat 

 during calm weather, might produce the effect, but pointed out 

 that the question is still an open one, and well calculated to repay 

 any labour bestowed upon it. To show how little is really known 

 of "the extent to which animal life is capable of being carried on 

 imder the widely-varying pressures occurring near the surface 

 and at great depths, he mentioned having repeatedly seen large 

 turtle " caught napping " at the surface in the Bay of Bengal, 

 several hundreds of miles away from the nearest point of land, 

 and where the sea was many hundreds of fathoms in depth. 

 These turtle must necessarily descend to the bottom to feed, if 

 they feed at all. He also drew attention to the circumstance that 

 their carapaces were studded with minute living algse, diatoms, 

 and foraminifera, the latter belonging, in some instances, to 

 sessile families, such as the Miliolidae. 



The Coscinodiscus referred to, and which has been described 

 and flgured by Dr. Wallich under the name of C. Begins* is pro- 

 bably the largest known diatom, the frustule measuring ^V^^ ^^ 



* One or more mounted specimens will be found in the Cabinet presented 

 to the Society. 



