383 



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 con- 

 dition of soft mud, must have found its way through the aperture in the 

 upper side into the great central cavity, and thence through the endorhi- 

 nal pores into the spaces between the tubes. In general the upper side or 

 vault, as it may be called, over the central cavity is not preserved, and 

 the specimens then consist of the whole or a portion of the base with the 

 nucleus, as in fig. 355. These are also filled with matrix ; the soft ooze 

 having entered not only through the pores, but also through the broken 

 margin. It is probable 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 exciting. 

 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 Avould be soon scattered, while the partly buried base would 

 be preserved. 



The specimen represented by fig. 358 is a fragment of the ectorhin of 

 R. calciferus, from the Calciferous formation, Mingan 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 Receptaculites for several 

 years after its discovery, until a large portion of the base of an individual 

 of R. 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. 359 is a cast of the inner surface of the ectorhin of R. Oweni (Hall), 

 from the Lower Silurian rocks of IlHnois. 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 be seen that there is, 

 betAveen every two of the orifices, a point where two of the lines cross 

 without an orifice at their intersection. Each one of these marks the 

 point where the angles of four plates met. Four stolons also met at each 

 of these points. This will be readily understood by comparing fig. 357. 

 In specimens in this state of preservation we see no traces of the sutures 

 between the plates, as the whole substance of the ectorhin — plates, sutures, 

 and all — is destroyed. 



