54 THE LESSON OF EVOLUTION 



Kadiolarians and sponge spicules. The figures of the 

 Foraminifera seem more doubtful. 



The Radiolarians are very minute, about one- 

 fifteenth of the diameter of similar forms in Cambrian 

 and younger rocks. Most of them have a thin 

 spherical shell pierced with holes, and are sometimes 

 furnished with spines ; but the forms are various. 

 Twenty- four genera have been distinguished, two- 

 thirds of which are still living ; and there are many 

 others, the genus of which cannot be determined, 

 although they are unquestionably Kadiolarians. By 

 far the commonest forms belong to Cerosphcera, a still 

 living genus, known also in the Silurian period, which 

 belongs to the legion Spumellaria, the fundamental 

 form of which is spherical or ellipsoid. But the 

 legion Nassellaria, in which the fundamental form is 

 ovoid, is also represented by nine genera, although the 

 individuals are not so numerous as those of the first 

 legion. 



Sponge spicules are probably common, but they are 

 generally broken. They belong chiefly to the simple 

 forms of Monactinellidse, or to the Lithistidas, or the 

 Tetractinellidae ; but a few fragments belonging to the 

 Hexactinellidae have been recognised. Many are of 

 branched or radiate type, and they are surrounded by 

 pyrites, which probably represents the sponge. The 

 fauna is, therefore, an extensive and varied one ; and it 

 is evident that both Eadiolarians and sponges had 

 existed for a long time when the rocks of Brittany 

 were being laid down. Even if they are wrongly 

 referred to the Huronian period, this great variety of 

 form may be taken as good evidence that the ancestors 

 of these Eadiolarians and sponges existed long before 



