GENUS NUMMULINA. 275 



prolongations of the chambers of the later whorls do not extend over the central portion of 

 the earlier, but leave its spire exposed. This is the case with some of the largest existing 

 Nummulrna;, — those of the Australian coast; between which and the OperciiJina of the same 

 seas, there is a continuous gradation. And the closing-in of the last whorl in the adult, which is 

 affirmed by MM. D'Archiac and Haime to be the characteristic distinction oi Nummulina, can no 

 longer be so regarded, when we find that not only does the same closure occasionally occur in 

 O'percnliiia (^ 447), but that, in an indubitable NirnniiiiJlna, the last whorl opens out as rapidly as 

 that of the most widely spreading Opercidina (^456). The large "boss" of solid shell-substance 

 which occupies the umbilical region on either side in many forms of the recent N. jjlanidata 

 (^ 456), filling up the space which would otherwise be left by the non-advance of the alar 

 lobes to the centre, exactly resembles that which we have seen in Atnpldsteghia. Although 

 the " reticulated " Nummiiliua differ widely from Orhiioides in their plan of growth, yet they 

 present a marked approximation to that type in one character which is of considerable importance, 

 namely, — the interposition of multiple flattened and isolated chamberlets between the successive 

 lamellse which intervene between the camerated median plane and the lateral surfaces. If Plate 

 XVIII, fig. 10, be compared with Plate XX, fig. 1, it will be seen how strong a resemblance 

 is thus imparted to the vertical sections of the two organisms ; and if the condition of the 

 segments of sarcode that occupy the reticulations into which the spaces between the .spiral 

 lamellae are divided in N. Garansensis (Plate XVIII, fig. 3), be compared with that of the 

 segments occupying the flattened polygonal chambers which are superposed one upon 

 another in Orhifoides MaidcUi (Plate XX, fig. 4), it will be seen that there is a strong simi- 

 larity in the two cases. In fact it is difficult to conceive how these reticular spaces can be 

 formed in Nnmmulina in any other way than the superficial flattened chambers of Orbitoides, — 

 namely, by the extension of the sarcode from the tubes of the surface immediately subjacent, 

 since their cavities are no less completely cut off from the chambers of the median plane in 

 the former case than they are in the latter. 



467. Geographical Distribution. — Notwithstanding the extraordinary development of the 

 Nummulitic type within a comparatively recent geological period, it is now represented only by a 

 small number of forms, all of them referable to the type N. plamdata, occurring in arctic, tem- 

 perate, and tropical seas, and for the most part of very humble dimensions. The biconvex variety 

 found on our own coasts, N. variolaria, was first described as an existing NummuUte by Prof. 

 Williamson (ex, p. 37) ; its diameter usually ranges between 1- 12th and 1-1 6th of an inch. The 

 tropical variety, N. radiata (the Naidilns radiatm of Fichtel and Moll, who obtained it from the 

 Red Sea), attains on the Australian shores a diameter of l-5th of an inch ; while the diameter of 

 its Philippine specimens,* of which the younger examples seem exactly conformable to the 

 preceding, is increased in one direction by the spreading-out of their last whorl to more than 

 l-4th of an inch. This form belongs to the group with radiating septal lines, which pi-esents 



* This remarkable recent form of Nummulina, fully described by me (xv) under the erroneous 

 designation Amphistegina Cumingii, whilst agreeing with the typical Nummulinee in every essential 

 particular, differs from them only in possessing a central boss of non-tubular substance like that of 

 Amphistegha, and in the opening-out of its last whorl like that of Operculina. 



