THE FORAM1NIFERA 



53 



or, in some cases, chitinous nature, which are secreted in the sub- 

 stance of the protoplasm in the neighbourhood of the nucleus. Passing 

 to the surface, they are there built together into a regular test. With the 

 exception of some forms 

 of Biloculina, which have 

 been found when living 

 at great depths with a 

 siliceous shell instead of 

 the normal calcareous one, 

 this is almost the only 

 instance of the secretion 

 by the Foraminifera of a 

 siliceous skeleton a fact 

 which is all the more 

 remarkable in considera- 

 tion of the prevalence of 

 siliceous skeletons among 

 the Radiolaria and He- 

 liozoa. 



CtA 



Fio. 3. 



Euglypha alveolate. Two stages in the process of division. 

 In one the nucleus is about to divide, and the plates which 



In most of the other 

 orders of Foramini- 

 fera, though a chitinous 

 element is present in 



t>A aL-AWnn if ia /!TT In one the nucleus is about to divide, and the plates which 

 SKClCtOn, It IS Only wm form the new shell are seen in the protoplasm ; in the 

 the basis in which Car- other the division is nearly complete. c.v, contractile 

 vacuole. (After Schewiakoff, 48.) 



Donate of lime, together 



with a small proportion of carbonate of magnesium and traces of 



other salts, are deposited. 1 



1 The following analyses are given by Brady (3, pp. xvii. and xxi.) of the tests of 

 two species one porcellanous, belonging to the Miliolidea ; the other perforate (see 

 footnote, p. 54), a member of the Nummulitidea : 



Orbitolites complanata, var. ladniata. 

 Silica ..... ... 0*14 



Carbonate of lime . . . . 88 '74 



Carbonate of magnesia . . . . 9 '55 



Alumina with phosphate of lime and magnesia . . \ occasional 



Alumina and ferric oxide . . . . / traces 



Silica 



Amphistegina lessonii. 



Carbonate of lime with a little organic matter 

 Carbonate of magnesia .... 

 Alumina with phosphates of lime and magnesia . 

 Ferric oxide 



98-43 



0-30 

 92-85 

 4-90 

 1-95 

 trace 



100-00 



Sollas has examined the specific gravity of the shells of perforate and imperforate 

 species of Foraminifera (66, p. 374), and finds that in perforate forms it varies from 2'626 

 to 2*674. In examples of Mttiola, Peneroplis, and Orbiculina it varies from 27 to 



