GENUS OPEECULINA: INTEENAL STEUCTUBE. 27 



will be seen that much variety presents itself; thus in Fig. VIII. A (as in fig. 2, Plate V.) 

 we observe 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 smooth of an inch. 



158. The branches of the interseptal canals which proceed towards the spiral laminae 

 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 cavity of the chamber, a circumstance which may perhaps be attributed to the 

 relatively smaller size of these passages in Operculina, 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 rale) form a network in the spiral lamina over the entire surface of the 

 chambers. 



159. 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. CARTER'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. 15 c, c) may be 

 seen in a vertical section which divides 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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