1865.] BILLINGS — SILURIAN AND DEVONIAN FOSSILS. 189 



other at an angle of 45°. The stolons run along the inner sur- 

 face of the ectorhin, but the endorhinal canals are excavated in the 

 substance of the endorhin. The space between the tubes is almost 

 always filled with the rock of the same kind as that in which the 

 fossil is imbedded. In perfect specimens, the rock, while it was 

 still in the condition of soft mud, must have found its way through 

 the aperture in the upper side into the great central cavity, and 

 thence through the endorhinal pores into the spaces between the tubes. 

 In general the upper side or vault, as it may be called, over the cen- 

 tral cavity is not preserved, and the specimen then consists of the 

 whole or a portion of the base with the nucleus, as in figs. 3, 4. 

 These are also filled with matrix ; the soft ooze having entered not only 

 through the pores, but also through the broken margin. It is prob- 

 able that the animal lived with its base partly buried, a portion of 

 the vault with the aperture projecting above the surface of the mud. 

 During the life the central cavity was perhaps kept free from 

 sediment by currents of water which the animal had the power of ex- 

 citing. But as soon as the currents ceased (with vitality), the mud 

 would enter freely. The vault would also soon fall to pieces, and the 

 fragments of all those individuals of which it (the vault) projected 

 above the surface of the sediment would be soon scattered while 

 the partly buried base would be preserved. 



The specimen represented hy fig. 6 is a .fragment of the 

 ectorhin of R. calciferus, from the Calciferous formation Mino-an 

 Islands. It shows only the inner surface on which the stolons 

 are still preserved, but the tubes are worn nearly to their bases. 

 It is rare to find specimens in that condition; and this one was 

 not suspected to be a RecejptacuUtes for several years after its 

 discovery, until a large portion of the base of an individual of 

 E. occidentalis was found, which, by having been slowly weathered 

 down from the upper side, retains the tubes over an area of several 

 square inches, while a considerable space around the nucleus 

 is covered with the squares formed by the stolons, precisely as 

 in this example. Fig 7 is a cast of the inner surface of the 

 ectorhin of E. Oweni (Hall), from the Lower Silurian rocks of 

 Illinois. The integument itself is totally removed. The vertical 

 lines are the impressions of the radial stolons, while the more 

 irregular and slightly curved transverse lines are the imprints 

 of the cyclical stolons. The dark points are the apertures of the 

 cylindrical perforations in the rock once occupied by the tubes 

 now entirely removed. On following any one of the lines, it will 



