24 DE. CABPENTEE'S EESEAECHES ON THE FOEAMINIFEEA. 



orifices of the tubuli are separated by a very delicate areolation (fig. 9#), as if the whole 

 substance were composed of an aggregation of prisms with a tubulus in the centre of 

 each. And this circumstance appears to me to afford the explanation of an observation 

 which I have elsewhere recorded in regard to certain fossil specimens of Nummulites 

 (loc. tit.), namely, that where the characteristic tubular structure is wanting, an appear- 

 ance of prismatic structure is occasionally presented*; for it is not by any means diffi- 

 cult to conceive that the same molecular change which obliterates the one feature, 

 should bring the other into increased distinctness. The size of the tubuli of Opercu- 

 lina is much 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 figs. 1 and 3 of Plate IV. 

 with figs. 2 and 4 ; 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 areolse, 

 as is shown in Plate IV. fig. 9 a, which is drawn on the same scale as b. 



155. As is Nummulites and Cycloclypeus, 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 IV. figs. 10, 11 ; 

 Plate VI. figs. 1, 2). 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 presents the orifices of the tubuli regularly disposed over the interior of the 

 chambers (as seen in Plate IV. figs. 1, 3), even where sections passing at a little distance 

 from these show rounded or elongated spots of transparent substance (figs. 2, 4), around 

 which the tubuli are crowded together, as if they had been displaced by its inter- 

 position. Such spots occasionally present themselves in the ordinary type of Operculina, 

 as seen in Plate IV. fig. 10 ; but it is of course in sections of the strongly-tuberculated 

 variety that we find them most marked, and this especially in the investing layers of the 

 central region, where, from the size and approximation of the tubercles, the spaces 

 between them are rendered almost opaque by the crowding together of the tubuli 

 (Plate IV. fig. 11). The tubuli are generally seen, moreover, to be somewhat more 

 crowded in the neighbourhood of the septal bands. The greatest local development 

 of the non-tubular substance is seen in those varieties which have a large elevated 

 tubercle in the centre (Plate III. figs. 3-5) ; a section of such a variety has been shown 

 in Fig. VII. D, where it is seen that this tubercle springs from the spiral lamina of the 



* As MM. D' AECHIAC and HAIME, in referring to my statement as to the appearance of prismatic struc- 

 ture in Nummulites, would appear to entertain some doubt as to the correctness of my observation (see 

 p. 61 of their Monograph), I think it right not merely to say that fig. 12, PL IV. of my Memoir on Num- 

 mulites is a most exact representation of a section still in my possession, but also to mention that the ver- 

 tical fracture of well-preserved specimens of N. lavigata often presents an edge so strongly resembling that 

 of the prismatic shell-substance of Pinna when fractured in the same direction (though on a much smaller 

 scale), as to have led experienced microscopic observers to a belief in their similarity, before my discovery 

 of the minutely tubular structure of Nummulites. 



