130 



PROTOZOA. 



whole, and lie partly inside the central capsule as well as 

 outside it. In this group are forms like Acanthometm (fig. 

 30, a) ; but though capable of preservation, these forms, like 



Fig. 30. Recent Radiolaria. a, Acanthometra ; b, Actinomma (Haliomma), one of the 

 Polycystina, showing the siliceous test and radiating pseudopodia. 



the preceding, do not occur in the fossil state. Lastly, we 

 have a group in which there is a siliceous, fenestrated or 

 perforated, coherent shell or " test," which is usually fur- 

 nished with projecting spines. In this group are the micro- 

 scopic marine organisms, well known for their beautiful 

 glassy skeletons, to which the name of Polycystina was given 

 by Ehrenberg. 



Many forms of the Polycystina have been preserved in the 

 fossil condition, and the distribution of the Radiolaria in 

 time, so far as known, is thus identical with that of this 

 particular section of the order. The earliest 1 undoubted 

 remains of Polycystina occur in the Jurassic series, and 

 several well-marked types have been detected in the Chalk. 

 All the remaining fossil Polycystina are referable to the Tertiary 

 period ; and the two most famous localities in which they 

 occur are Barbadoes and the Nicobar Islands. In the former 

 island, in particular, is found a great deposit of sandstone and 

 marls, which rises to heights of over 1000 feet above the 

 sea-level, and which is fundamentally composed of the sili- 

 ceous skeletons of Polycystina (fig. 7). The "Barbadoes 



1 Some forms of Polycystina have been indicated as occurring in the Trias. 

 Some Carboniferous fossils have also been referred to this order, but these 

 supposed ancient Eadiolarians appear to be really of a vegetable nature. 



