258 



FAMILY NUMMULINIDA. 



and the form of the meshes is very inconstant, so that the appearance presented is rather 

 that shown in fig. 4. Moreover the appearances exhibited by transverse sections of this 

 marginal cord, highly magnified (fig. 5, a a), do not at all confirm the idea of a spicular 

 arrangement, hut are altogether conformable to the view I have expressed ; the most complete 

 confirmation to which is afforded by tangential sections, of which a very successful example, 

 — giving a beautiful view, not only of the canal-system of the " marginal cord," but also of 

 its communications with that of the septa, and of its relations M'ith the two principal spiral 

 canals to he presently described,- — is represented in fig. 7, a, a. In this it is clearly 

 seen how irregular is the disposition of the inosculating passages, whilst there is not a 

 trace of the spicular structure, which ought, if it existed, to be as well brought into view in a 

 section taken in this direction, as it is in the one shown in fig. 3, which is takeii in a plane 

 at right angles to it. The " marginal cord " is traversed on its external surface by longitu- 

 dinal furrows, which are sometimes very shallow (fig. 5), but sometimes dip down deeply like 

 those between the convolutions of the brain. These furrows usually form a kind of network 

 with fusiform interstices, whilst in other cases they run parallel to each other and inosculate 

 rarely. From the correspondence between their arrangement and that of the passages in the 

 interior, I am inclined to think that they belong to the same system with these, being, in fact, 

 canals not covered in by shell-substance, that communicate with the plexus within by 

 inosculating branches, whose apertures may sometimes be detected in the bottom of the furrows. 



445. The " marginal plexus " of canals communicates freely with the system of " inter- 

 septal canals" (Plate XVII, fig. \, <),(/), which are disposed between the two layers of the 

 septa. The distribution of these is well seen, not only in vertical sections which have 

 happened to traverse the septa as in Figs. XL, XLII, and in Plate XVII, fig. 7, but also 

 in specimens laid open by fracture in the same direction, especially after the canals have 

 been more distinctly marked out by the imbibition of a colouring liquid (Fig. XLIII, and 



Pig. XLIII. 



Septal planes of four specimens of OpcrcuUiia, showins); varieties in the disposition of the iiilerscptal canals. 



