1865.] BILLINGS — SILURIAN AND DEVONIAN FOSSILS. 187 



be called the cyclical stolons. The connection of all these differ- 

 ent parts may be bstter understood by studying the following 

 figures. 



c ^&x& 



2. A small portion of R. occidentalis showing the tubes. 3. A part of 

 the lower side of the same species showing the nucleus and ectorhin. 

 4. A fragment of the same, showing the endorhin, the pores at the 

 angles of the plates, and the nucleus, which, on this side, is usually 

 deeply concave. 



At the lower side of fig. 2, is shown the ectorhin and the aper- 

 tures in the hollow stolons, broken off in the specimen. The aper- 

 tures are slightly enlarged in the figure. In fig. 2, the usual 

 aspect of the central portion of the lower side of this species is 

 given. It will be seen that the greater number of the plates are 

 not truly rhomboidal, but approach the form of a spherical triangle 

 with two of the sides concave. This form of the plates fre- 

 quently occurs. The convex side of such plates is always 

 outwards towards the periphery of the fossil, and the acute 

 angle formed by the two concave sides always directed towards 

 the nucleus. This shows that the consolidation of the plates 

 commenced at the nucleus and gradually extended outwards. In 

 many specimens the plates are all perfectly rhomboidal, and in such 

 they may have solidified simultaneously all over the body. One 

 corner of the specimen at a is represented as denuded of the ector 

 hin, showing the various markings beneath, which will be hereafter 

 explained by other figures. By fig. 4 is represented the ordinary 



