100 



GEOLOGICAL BIOLOGY. 



known, do not develop any structure which would be Hkely 

 to escape disintegration and resolution in the ordinary process 

 of fossilization. But the other two orders of the class, Fora- 

 minifcra, Radiolaria, develop hard skeletons of lime or silica, 

 and great numbers of them are preserved in a fossil state. 

 The Infusoria (a higher class than the others, in the possession 

 of mouth and vibratile or contractile cilia) are not known to 

 exist in a fossil state, though now abundant under proper 

 conditions, and though most probably they lived in like con- 

 ditions back to earliest geologic time. Figures lO, ii. 



Coelenterata. — Of the Coelenterata the classes Spongia and 



Fig. 12. — Spongia. A stylospongia preentorsa Gf. sp. Silurian. (S. and D.) Ay vertical sec- 

 tion ; B, lateral view ; C, silicious skeleton, greatly enlarged. 



Anthozoa and the Hydroid Zoophytes (Hydrozoa) are repre- 

 sented. All of the orders of the Anthozoa have families 

 producing some hard parts, " corals," which are preserved in 

 the rocks, but in each order there are some families not devel- 

 oping calcareous skeletons, hence not preserved ; and in the 

 Hydrozoa (class) several orders and a few whole subclasses 

 (as the Lucernaridae, Siphonophora, etc.) are of such a nature 

 as to be w^anting in any geologic record, and therefore in so 

 far the history of the Ccelenterata is necessarily imperfect. 

 However, Corals are among the most abundant fossils, and 

 Graptolites (related probably to the Hydroid Polyps, or 



