GENUS OPERCULINA: EXTERNAL CHARACTERS AND INTERNAL STRUCTURE. 19 



chambers being thus modified, and the number of chambers in each whorl being dimi- 

 nished ; the septal bands, not merely of the earlier whorls, but even of the last-formed 

 portion of the shell, are raised into prominent tubercles ; and multiple rows of large 

 tubercles are seen between the septal bands. It is to this multiplication of smooth 

 tubercles composed of a variety of shell-substance which reflects light much more 

 strongly than the rest, that the more glistening aspect of this type is chiefly due ; and I 

 have not met in it with any instances of that general brownish coloration of the surface 

 which is the ordinary characteristic of the other. The most constant and remarkable 

 distinctive feature of this type, however, is the presence^of a large hemispherical cluster 

 of semitransparent tubercles in the centre of the spire (Plate III. fig. 10). 



149. Having had no difficulty in setting apart a large number of specimens agreeing 

 very closely with each other, and differing from the rest, in all the foregoing characters, 

 I should have arrived at the unhesitating conclusion that this type deserves to take rank 

 as a species of Operculina distinct from the preceding, were it not for the circumstance 

 that every here and there I met with an example in which the differential characters 

 were less strongly marked than usual. Thus in some individuals which preserve the 

 general proportions of the spire, the green coloration is wanting, the large central 

 cluster of tubercles is replaced by a single tubercle of extraordinary size, the septal 

 bands of the neighbourhood are not more tuberculated than in many examples of the 

 ordinary type, and the rows of tubercles over the chambers are either wanting alto- 

 gether, or are not more prominent than in many individuals of the preceding type. 

 This evidence of the negative value to be attached to the number or prominence of the 

 tubercles as a specific character, is confirmed by a curious fact of an opposite nature ; 

 namely, that in certain individuals we find them developed to an extraordinary and 

 obviously abnormal degree (Plate V. fig. 9). From the difficulty of deciding to which 

 type particular specimens are to be referred, I had been almost led to adopt the con- 

 clusion of the specific identity of this with the ordinary form ; when Dr. GOULD, of 

 Boston (N. E.), kindly placed in my hands some Operculina, which had been collected 

 on the coast of Japan by the recent American expedition to that country. These speci- 

 mens combined in so remarkable a manner the most distinctive features of the two types, 

 namely, the general form and proportions of the one, with the umbilical hemispheric 

 cluster of tubercles and the general abundance of tubercular elevations characteristic of 

 the other, as to remove all doubt from my mind with regard to their specific identity. 



150. Internal Structure. The study of the internal organization of Operculina may 

 be prosecuted in two modes ; by the examination, under sufficient magnifying powers, 

 of thin transparent sections taken in different directions ; and by viewing under a 

 low power, as opaque objects, fragments obtained by breaking the shell, especially (as 

 Mr. CARTER was the first to suggest) after these have been allowed to absorb carmine or 

 indigo by being placed upon water in which either of these colours has been rubbed 

 up. By the former method alone can certain minutiae of structure be detected ; but the 

 latter is extremely serviceable (as I found in my study of Nummulites, op. cit. ) in enabling 

 the observer to trace out the relations of various parts, which sections exhibit to him 



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