34 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [May 2, 



really so great as they seem, since the apparent interruption of a row 

 of chambers may be due to its passage above or below the plane of 

 the section. In the enlarged representation given in fig. 34, it is 

 shown that the septa that divide the chambers of the same row are 

 in reality double, like the septa of the Nummulite. 



When we obtain a very thin section of the investing layers of O. 

 Prattii, and submit it to a sufficiently high magnifying power, we see 

 that each division is penetrated by a number of apertures, of nearly 

 the same size and distance from each other as those of the shell of 

 Nummulite (fig. 33) . These apertures form a communication, there- 

 fore, between the successive spaces that exist between the overlying 

 layers ; and in this manner the animals inhabiting the chambers of 

 the medial plane may have drawn in nutriment from the surface. In 

 many of the specimens of this fossil which I have subjected to mi- 

 nute examination, the apertures in question are not visible ; in con- 

 sequence, it would appear, of the changes induced by fossilization. 

 Their apparent absence in the preceding species, therefore, by no 

 means indicates their real deficiency ; and as the two species otherwise 

 agree in almost every other respect than in their size and in the form 

 of the chambers of the central layer, I am disposed to believe that 

 the structure in question was common to both. 



Among the Biaritz specimens of this fossil are many which are 

 more or less contorted ; and some of them are twisted into an ephip- 

 pial form. These last, however, agree so precisely with the flattest 

 individuals in the characters of their minute structure, whilst the 

 transition from the one to the other seems to be effected by so many 

 intermediate forms, that I think that we can scarcely regard them as 

 specifically distinct. Another specimen in Mr. Pratt's collection, 

 however, appears to have had a very different mode of growth ; for 

 instead of being a circumscribed disk, it seems to have spread itself 

 irregularly in every direction, and to have been attached to the sur- 

 face of rocks, whose contour it has followed, like the Lepralia or any 

 other incrusting Zoophyte. A section of this, taken parallel to the 

 surface, could hardly be distinguished from that represented in fig. 

 32 ; but the vertical section (fig. 22) differs in the much greater number 

 of incrusting layers. This might be regarded as merely an indication 

 of greater age ; but the absence of definite size and form in the spe- 

 cimens in question appears sufficient to justify their separation as a 

 species distinct from the preceding. In neither case do we find any 

 very definite markings upon the surface of the fossil, which might 

 serve as a distinctive character. The aspect of that of Orbitoides 

 Prattii, as seen with reflected light under a low magnifying power, 

 is shown in fig. 37. The markings seem to correspond with the 

 divisions shown under a higher power in fig. 33 ; and I have not 

 been able clearly to trace any large punctations at all similar to those 

 displayed by the group of species I shall presently describe, though 

 I have sometimes suspected their existence. 



The resemblance of these bodies to Nummulites is sometimes 

 greater than that which is shown by the preceding specimens. Thus 

 in fig. 18 is shown a section of a small form which I have happened 



