G-ENTJS OPEECULINA: — INTEENAL STEUCTUEE. 
27 
will be seen that much variety presents itself; thus in Fig. VIII. a (as in fig. 2, Plate V.) 
we obsers^e two or more such trunks proceeding from each angle of the fissure, those 
of the same side frequently inosculating with each other ; in Fig. VIII. c (as in fig. 8 , 
Plate IV.) the two principal trunks on the opposite sides inosculate as they approach 
one another in their course towards the outer margin of the septum ; and in Fig. V. 
(page 21 ) this inosculation is seen to take place much nearer the inner margin of the 
septum of the last whorl, so that the greater part of the septal plane is occupied by the 
plexus formed by their interlacement. The interseptal canals of one septum are occa- 
sionally, if not invariably, connected with those of another by loops formed between 
the ramifications of those canals which extend along the alar prolongations of the septa 
towards the centre of the shell; such inosculating loops are seen at the centre of fig. 6 , 
Plate VI. Moreover, branches of the same system are generally found to penetrate the 
non-tubular shell-substance whenever it accumulates in any quantity ; these branches 
are at once distinguished from the ordinary tubuli by their much larger size, their dia- 
meter being commonly about 5 -^g^th of an inch. 
158. The branches of the interseptal canals which proceed towards the spiral laminse 
are sometimes continued onwards in the non-tubular shell-substance that constitutes the 
septal bands ; and they are also frequently seen to diverge from the septa over the walls 
of the chambers, — still, however, being usually invested by a layer of transparent shell- 
substance, as is well shown at e, fig. 2, Plate VI. This arrangement, again, corresponds 
with what I have described and figured in Nummulites ; but I have not been able to 
detect in Operculina the apertures by which in Nummulites these canals communicate 
with the ca\ity of the chamber, a circumstance which may perhaps be attributed to the 
relatively smaller size of these passages in O^erculina, and the consequent facility with 
which their apertures may be confounded with those of the ordinary tubuli. In fig. 2, 
Plate VI. is represented an appearance of areolation I have several times met with in 
the walls of the chambers, — though usually less conspicuously than in this instance, — 
which has made me suspect that the ramifications of the interseptal canals sometimes (if 
not as a regular rule) form a network in the spiral lamina over the entire surface of the 
chambers. 
169. The interseptal system of each whorl is connected with the marginal plexus of 
the preceding by a very remarkable arrangement, which seems to have altogether 
escaped Mr. Caetee’s notice. At each junction of the marginal cord with the tubular- 
substance of the spiral lamina, the orifice of a large canal (Plate IV. fig. 16 c, c) may be 
seen in a vertical section which dirides this canal transversely; and either of these 
canals may be clearly traced, in sections parallel to the surface, which have happened 
to pass through its plane, as at d, d, fig. 10, Plate IV., b, b, fig. 1, Plate VI., and very 
conspicuously in fig. 5, Plate IV., in which, by a fortunate accident, one of the spiral 
canals has been laid open in its entire course through the inner whorls, and can be 
traced nearly to the central cell. In the tangential section, fig. 7, Plate IV., the two 
canals can be seen running on the outside of what may be called the nucleus^ consisting 
of the central cell and of the chambers immediately surrounding it. The mode in which 
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