256 FAMILY NUMMULINIDA. 



(the umbilicus itself being occupied by a prominent tubercle of solid shell-substance), though 

 the general proportions of the chambers correspond with those of the preceding, the aire are 

 so little extended that the spiral laminjc of the successive whorls coalesce not only at the 

 centre, but at some distance around it. — After an attentive examination and comparison of 

 a large number of vertical sections, I feel justified in affirming that no importance can be 

 attached to the form and proportions of the chambers, whether shown in the relative 

 length and breadth of the septal plane, or in the degree in which their al?e are prolonged 

 over the enclosed whorls, as furnishing characters of specific difl'erence ; seeing that it is not 

 only found to vary gradationally when a sufficiently large series of specimens is compared, but 

 that it is often equally inconstant in different parts of the same individual. 



442. The spiral lamina, in typical specimens, is much thicker in the inner than in the 

 outer convolutions, that of the last whorl being always comparatively thin (Fig. XL) ; and a 

 careful examination of transparent vertical sections makes it apparent, that the greater thick- 

 ness of the spiral lamina of the earlier whorls is partly due to the prolongation of that of the 

 later whorls over them, and to its coalescence with them. I have not been able to satisfy 

 myself that the spiral lamina of the last whorl is thus extended over the preceding whorls; 

 and it has rather appeared to me to be merely applied to the margin of that which it surrounds. 

 The difficulty of certainly determining this point chiefly arises from the circumstance that 

 the spiral lamina of the last whorl is reduced in thickness in proportion to its extension, as if 

 in consequence of a limitation of the amount of calcareous matter employed in its formation. 

 The spiral lamina is made up of a variable number of lamellre of minutely-tubular substance 

 (1 58) ; and its tubuli may be traced continuously through all its lamellie, as shown in Plate 

 XIX, fig. 3. In examining extremely thin transverse sections of this tubular shell-substance 

 with a sufficiently high magnifying power, I have been able to perceive that the orifices of the 

 tubuh are separated by a very delicate areolation (Plate XVII, fig. 9), as if the whole substance 

 were composed of an aggregation of prisms with a tubulus in the centre of each, as is shown 

 under a still higher magnifying power at fig. 8, a ; and this accords well with the idea that 

 each prism is really a hollow spicule (like that of Amnihomctra, 118), formed around one of 

 the pseudopodia. The diameter of the tubuli is greater at and near the inner surface of the 

 walls of the chambers than it is at a little distance from them, as will be seen by comparing 

 fig. 1 3 of Plate XVII with fig. 12; so that when a thin lamina from the innermost part of 

 the wall is examined under a high magnifying power, the tubes are seen almost to fill the 

 areolae, as is shown in fig. 8, b, which is drawn on the same scale as a. 



443. The shell-substance over the septa is not penetrated by tubuli, and is consequently 

 far more transparent than the rest ; and it is in consequence of this difference in texture, that 

 the septal bands are so strongly marked on the external surface, as well as in sections parallel 

 to it (Plate XVII, figs. 12, 13, aa, a' a). The same is true, also, of the substance of the 

 tubercles, whether occurring as elevations of the septal bands, or upon intermediate parts of 

 the spiral lamina. In the latter case, the tubercles are seated upon inverted cones of the like 

 substance, which do not usually reach down to the inner lamina, so that the internal surface pre- 

 sents the orifices of the tubuli regularly disposed over the interior of the chambers (as 

 seen in fig. 13), even where sections passing at a little distance from these show rounded or 



