Dr. G. C. Wallich on the Diatomacea, 9 



figure (using both these dcsiguations in their broadest sense) 

 were comparatively scarce both as to species and individual 

 numbers, and presented characters distinct from their non- 

 pelagic congeners. 



In the contents of the stomach of SaJpa and Diplujoi, the 

 discoid examples were numerous, although not of large size, as 

 might be expected*. Among these may be named Coscinodiscus, 

 Eujmdiscus, Asterolampra, Asteromplialus, and Triccratium. 



Rhizoselenia was always abundantly detected, but nowhere so 

 profusely as in the mid-Atlantic, where the digestive cavities of 

 monstrous Salpce, measuring from six to seven inches in length, 

 contained little else. 



Temperature, within certain limits, has probably little to do 

 with the bathymetrical distribution of the pelagic Diatoms ; for 

 it is well known that, whereas in the equatorial regions the tem- 

 perature decreases with the depth, at a tolerably fixed rate, until 

 it becomes stationary (or only subject to slight variation) several 

 degrees above freezing-point, — in the Polar region the converse 

 process takes ])lace, the temperature increasing from above with 

 the depth, and approaching to the standard which is probably 

 universal near the bed of the ocean. 



The question to what extent the subsidence and deposit of 

 minute organic remains may be influenced by oceanic currents, 

 can hardly be considered as bearing on the present subject. It 

 will be a point for future investigators to decide, how far the 

 results observable in such a case can be rendered expressive of 

 their causes. Or, should this, in its literal sense, appear a 

 visionary hope, we may, at all events, expect, by examining 

 facts as presented, to augment our practical knowledge of the 

 sea, and, with it, our means of verifying other and more palpable 

 phtcnomcna. 



Again, it seems highly probable, from what has been adduced, 

 that these vast aggregations of minute vegetable life, and (what 

 is of equal value as affording collateral evidence of their presence 

 somewhere in the neighbouring depths) of the minute creatures 

 which subsist upon them, exist in difFerent vertical zones, which 

 are partly determined by atmospheric conditions, and partly l)y 

 peculiar idiosyncrasies of the organism upon which those atmo- 

 spheric conditions operate. 



In the present state of our knowledge of their life-history, 

 anything beyond a notice of the most easily recognized and 

 demonstrable of these conditions would be futile. Nevertheless 

 I shall endeavour to show that, as regards the Diatomacese, 



• The Coscinodiscus referred to as being so conspicuous in the Indian 

 wftters is probably tlie largest Uiatoni known, the valve at times measuring 

 one-tweutieth of an inch in diameter. 



