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 lamina? 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 structure 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 A. Cumingii 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 WILLIAMSON has pointed out) to a secondary deposit of 

 minute papillae 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 Amphi- 

 stegina that are described by M. D'ORBIGNY* under the names of A. Hauerina, A. ma- 

 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 Amphistegince 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. 



* Foraminiferes Fossiles de Yieune, pp. 207-209. 



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