LATTICE WORKERS, OR POLYCYSTINA. 105 



belong the " Polycystin Marl" of Barbados, the 

 Nicobar clay, and the Mediterranean Tertiary de- 

 posits. From the Mesozoic, or Secondary period, 

 many well-preserved species have been described. 

 All the main divisions of the Jura, both the upper 

 and the middle, and especially the lower, appear, in 

 certain localities, to be very rich in well-preserved 

 shells of fossil Polycystina. The number obtained 

 from the Palaeozoic or Primary formations is much 

 less than from the other two. A few species are now 

 'known from various Palaeozoic formations, and not 

 only from the Permian and the Coal measures, but 

 also from the older Devonian and Silurian systems. 

 An important fact is recorded by Haeckel as to the 

 connexion between these fossil and extant living 

 forms. He says that "amongst the Miocene Radio- 

 laria numerous species are not to be distinguished 

 from the corresponding living forms. On the other 

 hand, certain genera, which are rich both in species 

 and individuals (recent as well as fossil), present con- 

 tinuous series of forms, which lead gradually and 

 uninterruptedly, from old Tertiary species to others 

 still living, which are specifically indistinguishable 

 from them." 



The skeletons of the Radiolaria are so beautiful, 

 and of such a variety in modification of their form, 

 that they are deservedly held in high esteem as 

 objects for the microscope, even when not studied 



