NATURE OF SURFACE MARKINGS OF DIATOMACE^. 281 



FiQ. 79. 



that we should have recourse for the determination of the nature 

 of these markings ; and we cannot resort to better illustrations, 

 than those which are afforded by Isthmia (Fig. 78), Trieeratium 

 (Fig. 79), and Biddulphia (Fig. 81), in all of which the structure 

 of the valve can be distinctly seen under a low magnifying 

 power and with ordinary light. In each of these instances, we 

 see a number of areolae, rounded, 

 oval, or hexagonal, with intervening 

 spaces symmetrically disposed ; and 

 the idea at once suggests itself, that 

 these areolae are portions of the sur- 

 face either elevated above or depressed 

 below the rest. That the latter is 

 their true condition, is suggested by 

 the appearances they present when 

 the light is obliquely directed ; and 

 it may also be inferred from the as- 

 pect presented by these objects when 

 viewed by the black ground illumi- 

 nation (§ 61), since the areolae are 

 then less bright than the interven- 

 ing spaces, less light being stopped 

 by their thinner substance. More- 

 over, when a valve is broken, the 

 line of fracture corresponds to what, 

 this supposition, is its weakest 



on 



yoouoa ^ 



loooapopooooj 



TiHceratium favus : — A, side viewj b, from 



portion ; since it passes through the 

 areolae, instead of through the inter- 

 vening spaces, which last would be 

 the weaker portions if the areolae 



were prominences. But the most satisfactory proof that the 

 areolae are depressions, is perhaps that which is afforded by a 

 side view of them, such as may be obtained by examining the 

 curved edges of the valves in Isthmia ; this, it may be safely 

 affirmed, can leave no doubt in the mind of any competent and 

 unprejudiced observer, as to the nature of the markings in that 

 genus ; and analogy would seem to justify the extension of the 

 same view to the other cases in which the microscopic appear- 

 ances correspond.' Now, it would not be difficult to bring 

 together a connected series of Diatomaceae, in which the mark- 

 ings, still exhibiting the same general aspect, become more and 

 more minute, requiring for their resolution the use of oblique 



' It is considered by Prof. W, Smith, that this areolation is indicative of a cellular 

 structure in the siliceous envelope. But when it is borne in mind that the entire frus- 

 tule constitutes a single cell, such an hypothesis seems altogether inadmissible. The 

 Author would rather consider the markings in question as analogous to those which are 

 presented by the surface of many pollen-grains (Fig. 189), of whose single-celled nature 

 no doubt can exist; and in his researches on the Foraminifera, he has met with several 

 instances, in which the calcareous investments of those segments of sarcode which must 

 be considered as the representatives of single oeUs, are marked with a like areolation, 

 the areote being here unquestionably depressions, formed by the thinning away of the 

 envelope at those parts. 



