GENUS OPERCULINA. 251 



sented also in no less a degree by those whose umbilicus is flat or even depressed. The most 

 remarkable departure from the ordinary type is seen in a small group of specimens in which 

 the tubercles are not only extremely large and prominent, but are distinguished by their 

 opaque whiteness, which contrasts strongly with the semi-transparence ordinarily charac- 

 terising these prominences. But even this character has no diagnostic value ; for a specimen 

 which has the tubercles opaque in one part may have them semi-transparent in another ; and 

 as to the size of the tubercles, their degree of prominence, and the proportion of the entire 

 spire over the septa of which they occur, there is every degree of variety. In one of my 

 largest and flattest specimens, the central region is strongly tuberculated, but the tubercles 

 almost suddenly cease when the spire begins to open out, and the septal bands are thenceforth 

 as smooth as in the ordinary type. 



437. A less obvious but still very decided feature of individual difference, consists in 

 the presence or absence of papillary elevations hdwecu the septal bands. I have already 

 spoken (^ 433) of the frequent existence of symmetrically arranged spots, sometimes slightly 

 depressed, but more commonly elevated, that are distinguished from the rest by the semi- 

 transparence of their shell ; these spots are sometimes considerably enlarged, and their elevation 

 increased, so that they become prominent papilla), closely resembling the tubercles upon the 

 septal bands. Their size and disposition vary considerably. Sometimes they are small, 

 numerous, and scattered without any definite arrangement over the entire surface of the wall 

 of the chamber ; whilst in other cases they arc considerably larger, and form single, double, 

 or even triple rows between the septal bands. Another remarkable variety of external aspect 

 is produced by the elevation of the general surface into rounded eminences closely abutting 

 on one another (like the pustules of the skin in a case of confluent smalUpox), and distin- 

 guished from the preceding by the absence of any peculiarity in the texture of the shell. 

 That these and other analogous variations of surface-marking have no value as differential 

 characters is at once demonstrated, not merely by their gradational approximation in different 

 individuals, but by the fact that they are presented in very diS'erent degrees on different 

 parts of the surface of the very same shell ; the chambers of one part of the spire being 

 strongly marked by certain of these peculiarities, whilst those of another may only present 

 indications of them, and those of a third may be perfectly smooth. 



438. The collection of Mr. Cuming, however, furnished me with a group of forms which 

 are at once distinguished from the rest by their general physiognomy, and which, when their 

 characters are examined in detail, appear to be separated from them by well-marked dif- 

 ferences. Their aspect is much more lustrous, and their hue much whiter, varied, however, by 

 a tinge of green diffused in irregular patches ; the spire does not in the largest specimens 

 make above three turns, and it begins to open out sooner than in the type already described ; 

 the septa are usually considerably more convex anteriorly, and are also rather more distant 

 from each other, so that the interval between them is greater in proportion to the breadth 

 of the spire, the shape of the chambers being thus modified, and the number of chambers in 

 each whorl being diminished ; 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 



