FIRST FORMS OF LIFE. 



31 



FIG. 4. 



FIG. 5. 



animals devoid of that structure, and possessing a humbler 

 nervous system. In the latter are placed first the rayed ani- 

 mals (Radiata) ; then, on a higher platform, as in some de- 

 gree co-ordinate, the Articidata, or jointed animals, as crus- 

 tacea, insects, spiders, and the Mollusca, or pulpy animals, of 

 which oysters, cowries, and cuttle-fish are examples. 

 To all these, the Vertebrata are as a beautiful su- 

 perstructure upon a rustic basement, in the four 

 great classes of ascending rank Fishes, Reptiles, 

 Birds, Mammalia. 



In a general enumeration 

 of the organisms of the 

 Lower Silurians, we may 

 first notice the examples of 

 an order which is placed in 

 the lowest subdivision of 

 the animal kingdom Po- 

 lypiaria the creatures to 

 which we owe those vast 

 coral reefs by which the 

 course of the mariner is so 



Catenipora esckaroides, a often obstructed in tropical 

 Silurian Coral. seas. It was, perhaps, by 



some peculiarly early deve- 

 lopment of this family that the materials were 

 formed, out of which, in a reduced and modified 

 state, were formed the few limestone strata which 

 occur among the first sedimentary rocks. In the 

 early rocks of Snowdon, in the Bala limestone, in 

 the Llandeilo rocks, and others classed in this de- 

 partment, detached corals occur (cyathophylla, 

 favosites, &c.), but not nearly in such large quan- 

 tities as in higher strata. In this order are also to 

 be placed those Graptolites which form so character- 

 istic a fossil in the lowest zones of the system. 



We come to creatures comparatively well or- 

 ganized, and yet still within the lowest division of 

 the Animal Kingdom, when we speak of Crinoidea, 

 which might be described as a lowly kind of star- 

 fish, fixed on the top of a flexible stalk arising from 

 the sea-bottom. Numberless calcareous plates 

 enter into the composition of the stalk, body, and nite, or Nave 

 multitudinous tentacula or arms of the crinoid, 



Actinocri- 



