RHIZOPODA ] 3 



(fig. 2, 7) occurs in the chalk and chalk flints, and has been 

 described as a species of " Spirolinite." Many of the cretace- 

 ous Foraminifera contain a brown colouring matter, which 

 remains after the shell has been dissolved with weak acid, 

 and has been regarded as the remains of the organic substance 

 which once filled all the cells. 



The " calcaire grossier," which is employed at Paris as a 

 building-stone, contains Foraminifera in such abundance that 

 one may say the capital of France is almost constructed of 

 those minute and complex shells. 



But it is in the middle eocene, or "nummulitic period," 

 that the Ehizopods attained their greatest size, and played 

 their most important part. Wherever limestones or calcare- 

 ous sands of this period are met with, these Foraminifers 

 abound, and literally form strata which in the aggregate 

 become mountain masses. These " nummulitic limestones" 

 are found in Southern Europe, in Northern Africa, and in 

 India ; they also occur in Jamaica. The commonest form is 

 the Nummulite (fig. 2, 8), which occurs in the building-stone 

 of the Great Pyramid. The Nummulites were evidently 

 sedentary organisms ; and, in the large thin species, one side 

 is moulded to the inequalities of the sea-bed on which it 

 grew. 



Polycystinew. — The tertiary marls of Barbadoes afforded 

 to Ehrenberg an extensive series of novel and extraordinary 

 microscopic organisms, composed of silica, but foraminated 

 like the shells of the PJiizopods. The same forms, and others 

 similar to them, have been met with in the deep-sea mud of 

 the Gulf of the Erebus and Terror, and more recently in the 

 mud of the North Atlantic soundings. They are quite dis- 

 tinrt in form and character from most of the silicious-shielded 

 Diatomacca% but some of them resemble the Coscinodiscus and 

 Actinocyclus. No less than 282 forms, grouped in 44 provi- 

 sional genera, have been described. 



