386 



the tubes are much more slender. They here assume the form of the 

 elongated connecting spicula of the true sponges. Fig. 364 is a similar 

 section, through a specimen of R. lowensis from the Trenton limestone at 

 Ottawa. At a, the central cavity is distinctly shown, filled with the grey 

 limestone matrix, which has also found its way between the tubes in the 

 base of the fossil. The shaded portions b b are replaced by a reddish 

 magnesian spar. The under side of the specimen is deeply concave, and 

 the peripheral margin is so convex as to resemble a cylinder coiled into 

 a ring. The aperture in one specimen of R. Jonesi is rounded, and 

 resembles the umbilicus of an apple. 



The figures given by different authors of foreign species show a consi- 

 derable range of variation in the general form, and apparently also in the 

 structure of the body-wall. The details given in this paper have been 

 made out principally by the study of numerous specimens of R. occiden- 

 talis, which is undoubtedly congeneric with R. Neptuni, the typical form 

 of the genus. In others, such as Tetragonis sulcata and T. parvipora 

 (Eichwald), there appears to be a transition to species in which the ector- 

 hin was a soft coriaceous integument, not distinctly plated, although con- 

 nected with the interior by tubes or spicula. The genus Tetragonis, 

 instead of becoming obsolete, might be retained for some of the species 

 which have a structure different from that of R. Neptuni. 



As to the zoological rank of Recept acuities there yet remains much 

 diversity of opinion. At the present time the most ably supported view 

 is that which places the genus in the Foraminifera near Orbitolites. Seen 

 in this light, the diagram at the head of this paper would represent the 

 soft and not the hard parts of the animal. If this be the true interpreta- 

 tion, then we must suppose that outside of the ectorhin there was a layer 

 of shell, and another layer covering the endorhin, or lining the great cen- 

 tral cavity. 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 speci- 

 mens are imbedded, while all that is, in this paper, described as constitu- 

 ting 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 connverted 

 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 



