GENUS PLANORBULINA. 207 



as is shown in several of the marginal segments of Prof. Williamson's figure (ex, Fig. 120). 

 From the time that a complete ring of chambers has been thus formed around the original 

 spiral, the continued growth of this organism takes place upon the cyclical plan ; each new 

 chamber being formed (as a rule) by a tent of shell thrown over the interspace between two 

 chambers of the preceding annulus, with each of which it communicates through its oval 

 aperture ; whilst in its turn it forms two apertures which will communicate with the two 

 chambers to be formed in alternation with it in the succeeding annulus. Thus in plan of 

 growth Planorhulina bears a considerable resemblance to that of the simple type of OrhitolUes 

 (^ 157); but nothing can be more different than the te.xture of the shell in the two cases; 

 that of Planorbulina being the most coarsely porous of any of the Botalliies (Plate XIII, 

 fig. 14), so that its animal can send out large pseudopodia from every one of its segments ; 

 while that of Orbitolites is everywhere unperforated save by the marginal apertures. The 

 segments of the cyclically growing portion usually elongate themselves in a direction which is 

 oblique to that of the surface whereon they grow, sloping inwards towards the centre in such 

 a manner as more or less to cover-in the original spire, which is thus only partially visible on 

 its free surface, or may even be entirely concealed. The form of the segments is usually more 

 or less ventricose ; that of the entire shell will vary according to the degree of their oblique 

 elongation, the centre being depressed when the spiral is merely surrounded by the cyclical 

 growth, but being elevated when the spiral is covered by the inward extension of the cyclical. 



359. Now the spiral plan of growth in which Planorbulina always commences, may con- 

 tinue without giving place to the cyclical ; and thus are generated three forms, Truncaftdina, 

 Anomalina, and Planulina, which connect the typical Platiorbulince with the ordinary Rota- 

 lines. The former of these, TruncafuUna, has ordinarily a plano-convex shell, with a crenulated 

 margin, and is attached to sea-weeds, zoophytes, shells, &c., by its flat side, on which the 

 whole spire is apparent ; whilst on its unattached surface, which is really the inferior, the 

 earher portion of the spire is more or less overgrown by the later (Fig. XXXII, d, e, p. 20). 

 Its form, however, is liable to very great variation, in accordance with the place and nature 

 of its attachment, the depth of water, the character of the sea-bottom, and other less de- 

 finable influences; so that many plates of the great work of Soldani (ci) are devoted to the 

 figures of the very protean T. variabilis, the ordinary Mediterranean example of this type. 

 The shell is coarsely porous, and its surface in the older specimens is roughened by minute 

 tubercles. The apertural fissure, instead of being restricted to the umbilical margin of that 

 portion of the chamber-wall which forms the septal plane, is usually extended upon tlie inner 

 margin of the inferior lateral surface, so as to become visible from beneath ; this prolonged 

 portion of the fissure is not included by the succeeding chamber, and thus it happens that 

 some of the later chambers continue to retain a direct communication with the exterior ; 

 this, however, being closed by a subsequent deposit of shell-substance in all the earlier cham- 

 bers. The shell of Triincatulina may assume the form of a high cone, as in T. rrj'ulgens, a 

 northern form, of moderate depths, distinguished by its smoothness and polish ; but the real 

 apex of the spire is still on the flat surface, where the whole spire continues visible. On the 

 other hand the shell may be extraordinarily flattened, so as to become scale-like and nearly 

 symmetrical, with squared edges and elevated septal lines, as in the variety described by 

 D'Orbigny as Plai/uUna Aritnincnsis (Fig. XXXI1,f) ; this presents so close a conformity to 



