LITUOLIDAE 89 



Sub-family SILICININAE 



Apart from certain fossils of which we have little personal experience, the existence of 

 really siliceous Foraminifera has always appeared to be a matter of uncertainty. A 

 statement by Brady (B. 1884, FC, pp. 100, 131) is probably responsible for the opinion 

 expressed by Lister (L. 1903, F, p. 53) and Cushman (C. 1928, F, pp. 11, 144, 

 148) that tests of nearly pure silica may be developed under deep-sea conditions. 

 Writing of the organisms found at Challenger St. 238, in the North Pacific, at a depth 

 of 3950 fathoms, Brady says: " Miliolae were the only representatives of the calcareous 

 forms, and the shells of these were no longer calcareous, but consisted of a thin film of 

 homogeneous silica, unaffected by acids, and iridescent when first taken out of spirit. . . . 

 A few Miliolae. . .were found to be unaffected by acids, and, upon further examination, 

 it became apparent that the normal calcareous shell had given place to a delicate homo- 

 geneous siliceous investment ". It would appear from this that Brady had actually dealt 

 with these specimens; but Heron-Allen (H.-A. 1915, RPF, pp. 264 and 272) states, on 

 the authority of Sir John Murray, that the specimens examined on board the ' Challenger ' 

 were not preserved, and that Brady's statement was made on the strength of Murray's 

 notes, and not from his personal knowledge of actual specimens. 



It is almost certain that the objects seen by Murray were chitinous linings such as are 

 common to all Foraminifera. At any rate, so far as we are aware, no one has subse- 

 quently described recent siliceous Foraminifera, although chitinous specimens have 

 been observed in both deep and shallow waters by many persons, ourselves included. 

 The discovery of truly siliceous Foraminifera in recent dredgings is therefore a matter 

 of more than ordinary interest. 



In the first place let us define the term "siliceous" as meaning " capable of resisting 

 the action of strong acids without structural change". There are many Foraminifera 

 belonging to various groups which, on superficial examination, might be considered 

 siliceous, but which will not withstand this test. All the Astro rhizidae and Lituolidae 

 make more or less use of siliceous particles in the construction of their agglutinate tests, 

 encrusting the chitinous membrane, which forms their basic structure, with sand grains 

 embedded in a cement secreted by the animal, which contains varying proportions of 

 silica, ferric oxide, and carbonate of lime. Moreover, many species of Miliolidae are in 

 the habit of encrusting their normally calcareous tests with sand grains of varying sizes, 

 often in such abundance as to conceal the calcareous structure. But the apparently 

 siliceous tests of the Miliolidae are instantly dissolved with effervescence on the addition 

 of acid, while the ferruginous cement of the agglutinate forms, almost without exception, 

 breaks down under the prolonged action of the acid. It is true that Brady (B. 1884, FC, 

 p. 286) states that "in rare instances silica or some siliceous compound is employed, 

 either by itself or in conjunction with other mineral substances", but the only example 

 he gives is Reophax nodulosiis, of which he says : " The incorporating medium is more or 

 less siliceous, sometimes to such a degree that large specimens, half an inch or an inch 

 in length, preserve their form after all the calcareous and ferruginous constituents have 



