THE AGE OF INVERTEBRATES OF THE SEA. 67 



studied them more thoroughly than any other living naturalist, 

 finds them as silent on the subject as the sponges or the corals. 

 In a serifis of papers recently published in the Geological 

 Magazine^ ue remarks as follows : 



" We find that the larger number of genera made their first 



Fig. 59. — Spiri/ermucronatus (f2onn.i). Devonian. 



appearance during the Palceozoic periods, and since they have 

 been decreasing in number to the present period. We wil 

 leave out of question the species, for they vary so little that it 

 is often very difficult to trace really good distinctive characters 

 between them ; it is different with the genera, as they are, or 



Tig. 59^. — Athyris subtilita(\{6\\). Carboniferous. 

 u, •■ Exteriors, c, Interior, showing spirals. 



should be, founded on much greater and more permanent dis- 

 tinctions. Thus, for example, the family Spiriferiim "nchules 

 genera w^ich are all charar^'^.rised by a calcified spiral lamina 

 for the support of the brachial appendages ; and , however varied 

 these may be, they always retain the distinctive characters of 

 the group from th r first appearance to their extinction. The 



F 2 



