248 FAMILY NUMMULINIDA. 



tends to close in. As I shall show hereafter, I cannot admit the validity of the last distinction ; 

 and the difference between Operculina a-nd Nummulina comes to rest only on the general form 

 of the spire and the conspicuousness of its convolutions, — characters which seem by no means 

 sufficient for the separation of the former as a genus distinct from the latter. As, however, 

 they mark a difference in the respective physiognomies of these types which enables them to 

 be at once distinguished by their external conformation, it seems for the present desirable to 

 retain Operculina as a generic type ; more especially as it is very generally diffused at the 

 present epoch, whilst Nummulina proper is almost extinct. 



431. The minute structure of this type had been investigated by two excellent observers 

 previously to the appearance of my own description of it (XA'). Under the designation of 

 "an undescribed species of Nonionina^' VvqI. Williamson gave an account (cviii) of the 

 structure of a shell abounding in the Manilla sand, which, not merely from his figures and 

 descriptions, but from a comparison of the specimens which he has kindly enabled me to 

 make, I know to be one of the smaller forms of the Philippine Operculina. Mr. H. J. Carter 

 (xviii) still more minutely described the organization of an Operculina which he obtained in 

 great abundance on the south-east coast of Ai-abia, the shells coming up attached to the 

 grease of the sounding lead from sandy bottoms of between ten and twenty fathoms 

 depth ; and of the identity of this with a larger form of Philippine Operculina I am enabled 

 to speak, from an examination of specimens which Mr. Carter has obligingly transmitted to 

 me. Tiie rich store of material placed at my disposal by Mr. Cuming from his Philippine 

 collection, not only enabled me to prosecute my inquiries into the minute structure of this 

 organism upon specimens of unparalleled size and degree of development, but also enabled 

 me to bring together a great body of data for comparison as to the extent of variation which 

 it may undergo, alike in external conformation and in internal organization. And as the 

 information hence obtained has a most important bearing upon the study of the closely-allied 

 genus Nummulina, I consider it desirable to go into more minute detail on this point than 

 would be otherwise accordant with the general character of the present treatise. 



432. E.rternal Characters. — The shell of a fully developed Operculina of the ordinary type 

 (Plate XVII, fig. 1)* is a compressed spiral of about •25 inch in diameter, and about 015 inch 

 in thickness, consisting of between three and four convolutions gradually increasing in breadth ; 

 these are in general nearly flat, but are sometimes a little arched between their inner and 

 outer margins, and are sometimes depressed so as to present a slight concavit)', especially 

 near the outer margin of the last whorl. The chambers are about seventy-five in number, 

 commencing from a primordial spheroidal cell, and progressively increasing in dimensions with 

 the widening of the spire ; their septa have for the most part a radial direction, but they bend 

 backwards near the outer margin of each whorl ; and they are marked externally by bands 

 which are distinguished by their semi-transparent aspect from the dull brownish hue of the 

 general surface. These septal bands are commonly on the same plane with the intervening 

 portions of the shell ; but sometimes the walls of the chambers are a little arched between 



* For the better display of tlie internal structure, the ideal figure has been constructed on the 

 model, not of the ordinary compressed Operculina, but of a variety with a more turgid spire. 



