1865.] BILLINGS— SILURIAN AND DEVONIAN FOSSILS. 193 



All the space between the tubes was also a compact mass of shelly 

 substance similar to that of the Foraminifera. But not a vestige 

 •of any such shell has ever been discovered. The space between 

 the tubes is invariably filled with the same kind of rock as that 

 in which the specimens are imbedded, while all that is, in this 

 paper, described as constituting the skeleton is in the same mineral 

 condition as are the hard parts of the corals, crinoids and 

 molluscs found buried in the same beds. In the ordinary limestone 

 whenever the solid portions of the other fossils are replaced either 

 by calcareous spar or silica, or partly by one and partly by the 

 other, the skeleton of Receptaculites is always found converted 

 into the same mineral substances. And again in the magnesian 

 limestones where the hard parts of fossils are, in general, totally 

 removed, so that the cavities once occupied by them remain empty, 

 we find Receptaculites in the same condition. We have not the 

 tubes themselves, but only the cylindrical perforations in the rock 

 which they at one time filled, while the existence of the stolons is 

 only indicated by grooves such as those represented in figs. 7, 9, 

 10. These facts seem to prove clearly that the space between the 

 tubes was not filled with shell substance, but either empty, or 

 entirely, or partly full of soft matter, which was immediately dis- 

 sipated after the death of the animal, and its place occupied by the 

 soft mud in or on which the creature lived. Were it otherwise 

 we would now find the space in question a compact mass of 

 calcareous spar or amorphous silex, while the tubes (or cells as 

 they would be in that case) might be filled with limestone. 

 In the magnesian specimens the ectorhin seldom, if ever, 

 remains ; and in species with flat plates the form (of the plates) 

 can rarely be made out, the only markings on the surface being 

 the grooves of the stolons. But where the plates were deeply 

 concave the position of the sutures- is indicated by more or less 

 strongly elevated ridges enclosing rhomboidal depressed spaces 

 with a tube-cavity in the centre. Fig. 1 represents a fragment 

 of R. Canadensis in that state of preservation. The rhomboids 

 in this case are not the plates themselves, but only their impres- 

 sions. In describing such specimens, the tubes are sometimes 

 spoken of as having rhomboidal openings, but this is an error ; 

 the tubes when perfect, as can be proved by hundreds of specimens, 

 are not open at all, but completely closed, at one end by the 

 ectorhin and at the other by the'endorhin. They all, however, 

 communicate with each other through the stolons and endorhinal 

 canals. 



Vol. II. n • No. 3. 



