GENUS AMPHISTEGINA : — OEGANIZATION. 
35 
though traversed here and there by a stray tubulus resembling the straight parallel 
tubuli of the spiral lamina. I am inclined to attribute this difference in part to the 
smaller scale of the last-named species, which prevents any part of its shell from being 
far removed from the contact of sarcode ; the absence of a canal-system being analogous, 
under this point of view, to the absence of Haversian canals in laminae of bone which 
are thin enough to draw their nourishment directly from the nearest vascular surface. 
The smaller species may thus be considered as — so to speak — a degraded form of the 
larger; the other differences in its structm’e being of very subordinate value. These 
differences, however, serve for its recognition by external characters ; for whilst the 
number of radiating septal bands to be seen in H. CuTYiingii is never much greater than 
twenty, and in young specimens does not reach half that amount, that of the radiating 
septal bands in A. gibbosa commonly exceeds thirty ; besides which, the backward turn 
which these bands take, when they have passed the margin of the penultimate whorl, is 
far more striking in the last-named species than in that which I have been specially 
describing. The granular character of the surface in the neighbourhood of the mouth, 
which seems due (as Professor WiLLmisoN has pointed out) to a secondary deposit of 
minute papillse of non-tubular shell-substance, is another distinguishing feature of 
A. gibbosa. To this species I am disposed to refer all the three fossil forms of AmpM- 
stegina that are described by M. d’Oebigny* under the names of A. Hauerina, A. mm- 
millata, and A. rugosa, since these differ no more from each other than do the recent 
examples of the species, — their chief distinctions being based on the degree of their 
departure from bilateral symmetry, and on the limitation of the alar prolongations 
on the flatter side to the marginal portion of the included whorl ; characters, which I 
have shown to possess no constancy, and to be therefore quite valueless for systematic 
purposes. I have examined several large AmphistegincB from the miocene of St. Do- 
mingo, which seem to conform to the same type ; but in consequence of the alterations 
brought about by fossilization, I have not been able to determine satisfactorily whether 
the apparent absence of a canal-system indicates that it had no existence in the recent 
organism; so that the identification of the species must rest on the less satisfactory 
characters furnished by the number and direction of the septal bands. 
* Eoraminiferes Eossiles de Yienne, pp. 207-209. 
