THE LOWEK AND UPPKR SILURIAN AGES. 65 



imbedded in a paste of clear transparent limestone, or 

 rather calcareous spar, infiltrated between them. I 

 have examined great numbers of slices of these lime- 

 stones, ever with new wonder at the packing of the 

 organic fragments which they present. The hard 

 marble-like limestones used for building in the Silu- 

 rian districts of Europe and America, are thus in most 

 cases consolidated masses of organic fragments. 



In the next place, the animals themselves must have 

 difiered somewhat from their modern successors. This 

 we gather from the structure of their stony cells, 

 which present points of difference indicating corre- 

 sponding difference of detail in the soft parts. Zoolo- 

 gists thus separate the rugose or wrinkled corals and 

 the tabulate or floored corals of the Silurian from those 

 of the modern seas. The former must have been 

 more like the ordinary coral animals ; the latter were 

 very peculiar, more especially in the close union of the 

 cells, and in the transverse floors which they were in 

 the habit of building across these cells as they grew 

 in height. They presented, however, all the forms of 

 our modern corals. Some were rounded and massive 

 in form, others delicate and branching. Some were 

 solitary or detached, others aggregative in communi- 

 ties. Some had the individual animals large and pro- 

 bably showy, others had them of microscopic size. 

 Perhaps the most remarkable of all is the American 

 Beatricea,^ which grew like a great trunk of a tree 



 First described by Mr. Billings. It has been regarded as 

 a plant, and as a cephalopod shell ; but I believe it was a corai 

 allied to Cystiphyllnm. 



F 



