GENUS NODOSARINA. 163 



its relations in a section passing tlirouoh the median plane ; its position, however, is always 

 more or less outside of the axis of the spiral, so as to be nearest to the convex border. The 

 elevated septal bands or rows of tubercles, the central umbilical tubercle, the mar^^inal cauna 

 or culiro with its radiating prolongations, and the longitudinal ribs or scattered spines with 

 which the surface is beset, are all composed of non-tubular shell substance. No additional 

 feature of any importance is revealed by sections of the shell. Even in the most highly 

 developed examples of it, I have not detected the least vestige of a canal-system ; and the wall 

 of each chamber seems to be simply joined- on to that of the chamber which preceded it, so 

 that each septum is formed not of two laminae, but only of a single one. 



249. Thus the differences between these two extreme forms reduce themselves to these, — 

 that the axis of growth changes from a straight line to a spiral, that the divisions between the 

 segments cross that axis obliquely instead of transversely, and that the aperture leaves its exactly 

 central position to become excentric. Now, among the varied forms of the Cristellarian type, 

 we meet with every gradation between such as persist in the spiral plan of growth through 

 their whole lives, and such as, after forming one or more convolutions, thenceforth increase 

 along a nearly rectilineal axis, still having their aperture at the convex margin. From these 

 last we pass by an equally continuous gradation (exhibited in the varieties of the single British 

 species C. suharcuatulu , ex, p. 30) to forms in which the increase is from the first along a line 

 so slightly curved that the primordial segment remains terminal, as in D(?«ifa//«« ; it was to 

 these intermediate forms with a central aperture, that the name Marginidina was given by 

 D'Orbigny. In proportion as they approach the rectilineal mode of growth, they become 

 more and more like Nodosaria in their general aspect, and especially in the replacement of 

 the carina or cidtro of the convex margin by a set of longitudinal costa that repeat one 

 another at regular intervals around each segment, and are continued from one segment 

 to another, — this being especially the case in those varieties in which the compressed form 

 has most yielded to the conical or cylindrical, whilst the costse along the two margins are 

 usually the strongest in the compressed forms, which thus show an interesting link of tran- 

 sition to certain examples of that modification of Nodosaria which has been distinguished 

 as' Lingulina (Plate XII, fig. 1). In Bimorphina (the true nature of whose growth was entirely 

 misunderstood by M. D'Orbigny) the early growth is truly Cristellarian, though the form of 

 this part of the shell is somewhat spheroidal ; but the direction of the axis suddenly becomes 

 rectilineal or nearly so, and the aperture, from being on the convex margin, becomes central, 

 so that the plan of growth is thenceforth truly Nodosarian. 



2.50. Returning now to the Nodosarian or rectilineal type, we find that this is subject to 

 like variations, both as to general plan of growth, and as to minor details. Those varieties in 

 which the axis is slightly curved, the segmental divisions somewhat oblique, and the aperture 

 shghtly excentric, — its displacement, however, being towards the concave margin, — have been 

 distinguished by the generic name Dnilalina (ex, figs. 40 — 44). The variations which have 

 been mentioned as presenting themselves in the form and mode of connexion of the segments, 

 and in the superficial inequalities, of Nodosaria, so precisely repeat themselves in Bentalina 

 (compare together figs. 38 and 42 of ex, or Plates I and II of lxxiii), as to excite surprise that 

 these two types should ever have been separated, — especially since it is made perfectly clear 

 by the comparison of a sufiicient number of forms, that a complete gradation exists between 



