NUMMULINTDA \ OPERCULIXA. 509 



I its aperture, which is here a single fissure at the inner edge of the 

 septal plane (Plate xv., fig. 19), and in the absence of the 'retral 

 [processes' of the segments of the Sarcode-body, the external walls 

 of the chambers being smooth. This form constitutes a transition 

 to the ordinary Nummuline type, of which Polystomella is a more 

 aberrant modification. 



391. The Nummuline type is most characteristically represented at 

 the present time by the Genus Operculina ; which is so intimately 

 united to the true Nummulite by intermediate forms that it is 

 not easy to separate the two, notwithstanding that their typical 

 examples are widely dissimilar. The former Genus (Plate xvi., 

 fig. 2) is represented on our own coast by very small and feeble 

 forms ; but it attains a much higher development in Tropical seas, 

 where its diameter sometimes reaches l-4th of an inch. The Shell 

 is a flattened Nautiloid spire, the breadth of whose earlier convolu- 

 tions increases in a regular progression, but of which the last 

 convolution (in full-grown specimens) usually flattens itself out 

 like that of Peneroplis, so as to be very much broader than the 

 preceding. The external walls of the Chambers, arching over the 

 spaces between the septa, are seen at b, b ; and these are bounded 

 at the outer edge of each convolution by a peculiar band a, termed 

 the Marginal Cord. This band, instead of being perforated by 

 minute tubuli like those which pass from the inner to the outer 

 surface of the chamber-walls without division or inosculation, is 

 traversed by a system of comparatively large inosculating Passages, 

 seen in cross section at a ; and these form part of the Canal 

 system to be presently described. The principal cavities of the 

 Chambers are seen at c, c ; while the Alar Prolongations of those 

 cavities over the surface of the preceding whorl are shown at c', c. 

 The chambers are separated by the septa d, d, d, formed of two 

 laminae of shell, one belonging to each chamber, and having 

 spaces between them in which lie the Interseptal Canals, whose 

 general distribution is seen in the septa marked e, e, and whose 

 smaller branches are seen irregularly divided in the septa d', d', 

 whilst in the septum d" one of the principal trunks is laid open 

 through its whole length. At the approach of each septum to the 

 marginal cord of the preceding, is seen the narrow fissure which 

 constitutes the principal Aperture of communication between the 

 chambers ; in most of the septa, however, there are also some 

 isolated pores (to which the lines point that radiate from e, e) 

 varying both in number and position. The Interseptal Canals of 

 each septum take their departure at its inner extremity from a 

 pair of Spiral Canals, of which one passes along each side of the 

 marginal cord ; and they communicate at their outer extremity 

 with the Canal-system of the marginal cord, as shown in Fig. 260. 

 The external walls of the Chambers are composed of the same 

 finely-tubular Shell-substance that forms them in the Nummulite ; 

 but, as in that Genus, not only are the septa themselves composed 



