120 PROTOZOA. 



skeletons in course of time underwent solution, and the siliceous 

 Sponges are now found in the White Chalk only in the form of casts, 

 or with their original skeleton replaced by some such substance as 

 peroxide of iron. The dissolved silica thus produced was ultimately 

 segregated from the mass of the Chalk, and is now represented by 

 the nodules of flint which form such a characteristic feature of parts 

 of the White Chalk. It would appear, therefore, that the White 

 Chalk in its original chemical composition must have been very 

 closely similar to the modern Globigerina ooze ; and it is a reason- 

 able conclusion from its general purity that it must, like its modern 

 analogue, have been deposited in a deep and open ocean, at a con- 

 siderable distance from land. At the same time, the characters of 

 the Mollusca which are predominant in the Chalk, would go to show 

 that this remarkable Foraminiferal deposit must have been laid down 

 in general at depths less than those at which the modern Globigerina 

 ooze is now accumulated. 



In the Tertiary period, lastly, the remains of Foraminifera are 

 abundant and varied, and are often present in such numbers as to 

 give rise to more or less extensive Foraminiferal limestones. The 

 most notable of these is the great " Nummulitic limestone " of the 

 Eocene. In the later Tertiary deposits, the genera represented are 

 mostly those now in existence, and many of the species are identical 

 with living forms. The Foraminifera, in fact, like other lowly 

 organised forms, have been very " persistent " types of life. A 

 number of the Cretaceous species, indeed, appear to be inseparable 

 from existing forms, while some living species are believed to date 

 from even an earlier period. As regards the genera, the prevalent 

 types in Carboniferous times still survive unchanged ; and Dr Carpen- 

 ter has enunciated the conclusion that upon the whole " there is no 

 evidence of any fundamental modification or advance of the Forami- 

 niferous type from the Palaeozoic period to the present time." 



Classification of the Foraminifera. — The classification of 

 the Foraminifera has proved a matter of considerable difficulty. 

 The older arrangements were unnatural, as being based wholly on 

 the form of the shell, a point in which the Foraminifera show a most 

 marvellous variability. For this reason, the artificial systems pro- 

 posed by D'Orbigny and Max Schultze have now been generally 

 abandoned, and their place has been taken by other more natural 

 schemes of classification, and especially by those put forward inde- 

 pendently and almost simultaneously by Professor von Reuss upon 

 the Continent, and by Dr W. B. Carpenter, Mr Parker, and Professor 

 T. Rupert Jones in this country. Both these arrangements agree in 

 the essential feature that they divide the Foraminifera into two great 

 primary divisions, in accordance with the nature of the shelly invest- 

 ment. In the one division (Imperforata), the test is not perforated 



