170 Mr. H. J. Carter on the Form and Structure 



papillse with their puncta ; there are no large papillae on it, and 

 their cavities are otherwise complete, with the exception of the 

 channels of intercameral communication, and some minute vas- 

 cular apertures which will be presently mentioned. 



The septa (fig. 5 b, b) occupy, transversely, about l-6th of 

 the breadth of the chambers, and each septum incloses within 

 its walls two calcareous tubes or vessels, one on each side, some 

 little distance below the contiguous surface of the shell (fig.7«,«); 

 these we shall call interseptal vessels. They are irregular both 

 in their size and course, though generally about 1-1 900th of an 

 inch in diameter, in the last-formed septa of a shell having the 

 dimensions of the one described, and diminish in calibre back- 

 wards or towards the first- formed whorls. Each vessel com- 

 mences in the centre of an intricate network of smaller ones, 

 spread over its own side of the margin of the preceding whorl, 

 and under the layers of the shell (/,/,/) ; these networks, which 

 are joined together, we shall call the marginal pleomis. In its 

 course each interseptal vessel gives off two sets of ramusculi, and 

 the marginal plexus one set. Of those coming from the inter- 

 septal vessel, one set terminates on the surface of the shell, par- 

 ticularly about the borders of the septum {d,d) ; the other goes 

 into the walls of the shell, and through the septum, to open 

 probably on the inner surface of the chamber {e, e, e) ; while the 

 set from the marginal plexus opens on the margin (^, g^ g). As 

 this vascular system appears to extend throughout every part of 

 the shell, and must be for the cii*culation of some fluid, we will 

 call it the interseptal circulation. It would have been more pro- 

 per to have commenced with the ramusculi, as we shall see here- 

 after that they appear to absorb the fluid which is subsequently 

 transmitted into the larger vessels, but at this period of our de- 

 scription it would not have been so intelligible. 



We have now to examine the internal structure of the shell, 

 and commencing with that part forming the walls of the cham- 

 bers, we observe, that it is pierced by innumerable tubes, which 

 pass directly downwards from the small papillse on the external, 

 to the small papillse on the internal, surface of the chambers 

 (fig. 3 djd). I could see no tubes passing down from the large 

 papillse, which I have before stated to appear imperforate, like 

 those over the septal spaces. These tubes are about l-9000th 

 of an inch in diameter, and about the same distance apart ; they 

 are vertical over the centre of the area of the chamber, and slope 

 outwards at its boundaries, but do not pass through or extend 

 over the margin of the shell, neither over the septal spaces, nor 

 over the central cell; hence the semitransparency of the two 

 latter, and the fringy, beaded appearance which the tubes pre- 



