GENUS NUMMULINA. 273 



median plane (figs. -4, 6) ; and two more nearer to its outer edge, at a wider interval from 

 each other. These last, which seem to be the representatives of the spiral canals in Opercu- 

 liiia, sometimes lie close to the surface of the marginal cord, from which their outer M'alls 

 form a projection, as shown in fig. 7; whilst in other instances they seem to be merely 

 furrows in its surface of unusual depth, not covered-in externally, as has been noticed by 

 MM. D'Archiac and Haimc in N. planulata (i, p. 63, pi. ix, fig. 7), and by Mr. Carter ia 

 N. Iiaimo)idi (xxni, a, p. 311, pi. xvii, fig. 15). All the longitudinal furrows with which 

 the surface of the marginal cord is always strongly marked, arc probably to be regarded (as 

 in Operculina, ^ 444) in the light of canals not completed into tubes by shell-substance ; the}' 

 seldom present that inosculation by transverse or oblique furrows which is often to be seen 

 in Operculina ; but the orifices of the radiating canals are v'cry plainly to be distinguished in 

 them in well-preserved specimens, especially when the surface of the marginal cord has been 

 cleaned by dilute acid. It is evident both from vertical and from horizontal sections (figs. 2 

 and 4), that the radial canals which cross the marginal cord, and which establish a direct 

 continuity between the interseptal system of each whorl and that of the succeeding whorl, 

 constitute the most important feature of the canal-system ; this communication being quite 

 independent of the spiral canals, with which the radial canals here only incidental!}^ inoscu- 

 late. Thus the canal-system of NummuUna comes to bear a stronger resemblance to that of 

 Cydocli/pcus than to that of OjJerculbia ; the manner in which the interseptal systems of 

 successive convolutions are brought into direct continuity with each other by the canals that 

 cross the marginal cord, being almost identical in the two former cases (compare Plate XVIII, 

 fig. 4, with Plate XIX, figs. 6, 7), whilst in the latter there is no such continuity, the 

 "marginal plexus" of the cord itself afi'ording all the means of communication that exists in 

 addition to the spiral canals. Hence it seems fair to surmise that whilst in Operculina the 

 growth is continuously spiral, the increase being effected by successive gemmations from the 

 stolon that issues from the apertural fissure, the extension of large pseudopodial prolongations 

 through the radial canals of the marginal cord may take a considerable shave in the process 

 in NummuUna, so that its growth may be partly radial, as in Cijchchjpeus ; especially since this 

 indication harmonises in a remarkable degree with that which we have seen to be furnished by 

 the occasional bifurcation of the spire (^ 459). — Besides the radial and spiral canals, we find 

 branches from the interseptal system proceeding towards the two lateral surfaces, usually either 

 in the substance of the non-tubular columns (as shown at e, e, fig. 7) or in close contiguity to 

 them. These branches, however, do not seem to attain the number and importance which they 

 possess in some other instances. Other branches, again, occasionally extend themselves from 

 the interseptal system over the intermediate portions of the spiral lamina, here again being- 

 surrounded by non-tubular substance ; this, however, seems rather an abnormal than an 

 ordinary distribution. — The interseptal system of canals is continued through the alar prolon- 

 gations of the septa • and where these inosculate into a network, tlicsc canals form a complete 

 reticulation surrounding the subdivisions of the chambers, as shown in fig. 3. 



465. Varieties. — -It will doubtless seem strange to such Palaeontologi-sts as have made a 

 special study of Nummulites, and have not familiarised themselves with the wide range of 

 variation that characterises the group to which they belong, for mc to affirm that among all the 

 reputed qjecies of this type, I cannot satisfy myself that there are any which are entitled to a 



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