26 HENRY B. BRADY. 



The precise way in which the siliceous sand-grains are 

 held together and the nature of the cement, if there be any 

 true inorganic cement, are points not easy to determine ; 

 diiFerent genera probably differ widely in these particulars. 

 The very small percentage of calcareous matter in proportion 

 to the silica, in both of the above analyses, is a remarkable 

 fact, and one that scarcely accords with the idea that the 

 siliceous material is incorporated by carbonate of lime, as 

 generally set forth. 



The smooth tests of the various species of Trochammina 

 have been supposed to contain the largest amount of calca- 

 reous matter in proportion to siliceous or other embedded 

 constituents of any of the arenaceous Foraminifera ; but at 

 great depths, for example at 3000 to 4000 fathoms in the 

 North Pacific, specimens of Trochammina {Ammodiscus) m- 

 certa are found, the most delicate of which will bear treat- 

 ment with nitric acid without material change. Nor is this an 

 isolated fact. I have before me some specimens of Lituola 

 [Reophaw) iiodulosa, obtained near the Antarctic Circle 

 (Station 156, — 1975 fathoms), of large size, that is to say, 

 ranging from half an inch to an inch or even more in length, 

 and stout in proportion. One of these has been digested 

 for a considerable time in nitric acid with the assistance of 

 heat, until everything soluble was removed, yet it still re- 

 tains its form unimpaired, and has sufficient solidity to bear 

 handling ; indeed, the only change apparent to the naked 

 eye is the alteration of colour from dark brown to dirty 

 white. It is clear that in this case the cement was neither 

 calcareous nor ferruginous. In a later paper I shall be able 

 to show that, under certain conditions, true Ililiola are to be 

 met with in which the test is thin, homogeneous, and purely 

 siliceous — so completely siliceous that no effervescence can be 

 detected when the shells are placed in acid under the micro- 

 scope. It was long since demonstrated that the tests of 

 Trochammin(S , living in brackish water, are non-calcareous, 

 though they retain to a great extent their normal sandy 

 exterior, and that one of the Miliolae under similar conditions 

 has a thin, brown, sandy investment. In both these cases 

 the sand-grains are embedded in a chitinous envelope and do 

 not depend on any mineral cement. From such facts it 

 becomes evident that carbonate of lime and peroxide of iron, 

 though secreted to a greater or less extent by many arena- 

 ceous Ehizopods, are by no means necessarily the cementing 

 material ; — that a chitinous envelope or external layer of 

 altered protoplasm may be the basis to which the sand-grains 

 are adherent or in which they are more or less embedded; — 



