GENUS ORBICULINA: ORGANIZATION. 549 



other types, we shall be in a position to inquire what value is really to be attached 

 to such a character as a basis for classification. 



83. Organization. Among- the diversified forms that are presented by the species 

 before us, it is of course necessary to select some particular type as that with which 

 others may be compared; and this I consider to be the form delineated in Plate XXVIII. 

 fig. 5 ; since by far the larger proportion of the very numerous specimens I have 

 examined show such a degree of approximation to it, that their differences may fairly 

 be set-down to the account of incomplete development. The general aspect of a 

 typical Orbiculina, then, differs but little from that of Orbitolites, except in the pro- 

 minence of its nucleus, and in the peculiar spiral disposition of the bands by which 

 the surface of the nucleus is marked. The species never attains, so far as I am 

 aware, the dimensions of Orbitolites ; the diameter of the largest recent disk in rriy 

 possession (one of Mr. CUMING'S Philippine specimens) being -20 of an inch; whilst 

 of the specimens which I have seen from the West Indian and ^gean seas, none 

 exceed -12 of an inch. Mr. CARTER*, however, describes under the name of Orbito- 

 lites Malabaricus a fossil Orbiculina (^[ 90), whose specific identity with the type 

 before us I see no reason to doubt, as attaining a diameter of from 7 to 8 lines. The 

 thickness of the disk is usually less in proportion to its diameter, than it is in Orbi- 

 tolites; but this is by no means a constant difference, depending as it does merely on 

 the relative increase of the columns of sarcode in the vertical direction, as compared 

 with the extension of the surface by the addition of new annular bands. 



84. On more closely comparing the marginal portion of such a disk with the cor- 

 responding portion of one of those forms of Orbitolites, in which the concentric annu- 

 lation is strongly marked externally whilst the transverse divisions of each annulus 

 are scarcely indicated (^[49), no essential difference is perceptible between them; 

 and although these transverse divisions are usually but very faintly indicated in 

 Orbiculina (Plate XXVIII. figs. 13, 14), yet they are sometimes very obvious, as is 

 seen in figs. 15, 16. In the fossil Orbiculina just adverted to, the surface-markings 

 (figs. 21, 22) are entirely conformable to the ordinary type of Orbitolites. The 

 margin itself exhibits one or more ranges of pores (figs. 6, 7, 18, 19) arranged after 

 precisely the same fashion as those of Orbitolites: these ranges are more numerous 

 in the fossil than in the recent forms of this type. 



85. The internal structure of Orbiculina presents such a general conformity to that 

 of Orbitolites, that it will not be requisite here to do more than specify the points of 

 agreement and of difference. The general aspect of the horizontal section of such a 

 typical specimen as is shown in Plate XXVIII. fig. 1 1, does not differ from that of a 

 corresponding section of Orbitolites in any other essential particular than the dispo- 

 sition of the central portion, in which the successive additions to the first-formed 

 chambers are so made as to produce a spiral, instead of a concentric disk. When we 

 examine this central portion more closely, we see that its plan of growth is exactly the 



* Annals of Natural History, New Series, vol. xi. p. 425. 

 4 c 2 



