GENUS OPEECULINA: — INTEENAL STEUCTUEE. 
25 
innermost convolution, but that it increases in diameter as it is built upon (so to speak) 
by each of the whorls by which this is subsequently invested. 
156. The texture of the shell-substance which bounds the chambers at the outer 
margin of each whorl, however, presents a marked departure from that of the spiral 
lamina ; a difference analogous to that which was first indicated by me in Nummulites 
{Joe. cit.), and which has been found by MM. d’Aechiac and Haime to present itself so 
constantly in that genus, that they give to this marginal portion the special designation 
“bourrelet.” Mr. Caeter has akeady recognized in Ojperculina the speciality in the 
structure of this part of the shell ; but he has fallen into what I believe to be an error 
in his interpretation of that structure, describing it as made up of an aggregation of 
fusiform spicules, and hence designating it as “ the spicular cord.” Now I am very 
familiar with the appearance which has led him to take this view of its nature, since 
it is one which is commonly presented by thin sections that pass either through the 
median plane or parallel to it, as shown in Plate IV. fig. 13 ; but this is only one out of 
many aspects which are presented by sections taken in various directions ; and a com- 
parison of the whole seems to me to lead to quite a different conclusion,— namely, that 
the supposed spicular composition of this “ marginal cord ” (as it may be appropriately 
termed) is due to the peculiar manner in which the homogeneous substance of which it 
is composed is traversed by the set of canals that are correctly described by Mr. Carter 
as forming the “ marginal plexus.” For if the section should happen to traverse a por- 
tion of this plexus in which the canals form a tolerably regular network of elongated 
meshes in one plane, the appearance delineated in fig. 13 is presented ; but just as often 
a less complete layer of the network is traversed by the section, and the form of the 
meshes is very inconstant, so that the appearance presented is rather that shown in 
fig. 14. Moreover the appearances exhibited by transverse sections of this marginal 
cord, highly magnified (figs. 15, 16), do not at all confirm the idea of a spicular arrange- 
ment, but are altogether conformable to the \iew 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 with the two 
principal spiral canals to be presently described, — is represented in Plate IV. fig. 12. 
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 \iew in a section taken in this direction, as it is in the one shown in 
fig. 13, which is taken in a plane at right angles to it. The “ marginal cord ” is traversed 
on its external surface by longitudinal furrows, which are sometimes very shallow (fig. 15), 
but sometimes dip down deeply like those between the convolutions of the brain, as 
shown in fig. 16. These furrows usually form a kind of network with fusiform interstices 
(Plate V. fig. 5), whilst in other cases they run parallel to each other and inosculate 
more rarely (fig. 4). 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 
MDCCCLIX. E 
