GENUS NUBECULARIA. 71 



Sometimes, af^ain, a new chamber is formed at the side as well as at the end of the preceding, and 

 thus a branch may pass off at a considerable angle from tlie main axis (fig. 12). More commonly, 

 however, wlien such a lateral gemmation takes place, the new series of chambers which thus 

 originates advances along the side of the preceding (fig. 7) ; and by further offsets of the same 

 kind these parallel series may multiply to any extent, the new chambers sometimes arching 

 over those from which they are derived, so as even in these flat spreading forms to mark the 

 tendency to an acervuline aggregation. Furtlici-, the axis of growth may be neither straight 

 nor spiral, but may bend or twist in any direction ; the chambers still succeeding one another, 

 either in a single linear series, or in ramifying extensions, or in multiple rows ; and nothing 

 then marks the generic type save the texture of the shell, its attachment by one of its surfaces, 

 and the narrowing of the chambers behind the constrictions, followed by a dilatation in front 

 of them. 



90. Instead, however, of extending itself in length, the shell not unfrequently widens 

 itself out so much, that the distance across the chambers is far greater than that which inter- 

 venes between the successive septa ; and it is then to be observed that, between each chamber 

 and the next, there are two or more apertures in the septum that divides them (fig. 15). This 

 plan of growth is carried to an extraordinary extent in the specimen represented in fig. 13, in 

 which the enormous increase in the transverse dimensions of the chambers is accompanied by a 

 great multiplication of these septal communications (fig. 14), so that a NiibeciiJaria formed upon 

 this type becomes a sort of rude sketch oi Penerojdis. In not a few cases, again, the growth takes 

 place almost from the commencement on the cyclical ^\n.xi (fig. 11), the first-formed chambers 

 extending themselves around that which is occupied by the primordial segment, and budding 

 off new chambers in all directions ; the successive chambers communicate with each other 

 laterally, as well as in a radial direction ; and thus a sort of sketch is presented of the plan of 

 growth characteristic of Orhitolites and Plunorbulina, to the lower forms of which last, indeed, 

 this type of Nuheeularia (the chambers of which I have always found to be much smaller than 

 the average) often presents a decided analogy. 



91. Either of the foregoing modes of growth may give place to one in which no 

 regularity whatever can be traced, the successive chambers being no longer added on the 

 original plane alone, but piling themselves upon one another, without any discoverable system, 

 so as to imitate the acervuline growth which Planorbulina sometimes takes on. This is espe- 

 cially liable to happen with NuhccuJaricc that cluster around the stems and branches of 

 Corallines, Zoophytes, &c. ; and thus it comes to pass that, as already remarked, not only 

 does their external configuration become altogether amorphous, but their internal structure 

 seems to be entirely destitute of arrangement. By a careful comparison of the intermediate 

 forms, however, the true nature of these amorphous acervuline growths, as derivations from 

 the simpler and more regular types, is unmistakeably demonstrated. In the larger and 

 coarser acervuline specimens, it is not unfrequently to be observed that the surface is 

 roughened by the inclusion of fine particles of sand (which seem to be composed of commi- 

 nuted shell) in the proper calcareous substance of their shells ; but this substance is never 

 replaced by sand,, as in the truly arenaceous types. 



