18 
DE. caepentee’s eeseaeches on the foeajsiinieeea. 
very prominent (Plate III. figs. 3, 4, 5), usually show this feature in the most decided 
manner; but it is occasionally presented also in no less a degree by those whose umbi- 
licus 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 (Plate IV. fig. 6), but are distinguished by their opaque whiteness 
(Plate V. fig. 12), which contrasts strongly with the semi-transparency ordinaiil) chaiac- 
terizing these prominences. But even this character has no diagnostic value ; for a speci- 
men which has the tubercles opaque in one part may have them semi-transparent m 
another; and as to the size of the tubercles, their degree of prominence, and the pro- 
portion 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 begms to open 
out, and the septal bands are thenceforth as smooth as in the ordinary type. 
147. A less obvious but still very decided feature of individual difference, consists m 
the presence or absence of papillary elevations between the septal bands. I have already 
spoken (^ 142) of the frequent existence of symmetrically arranged spots, sometimes 
slightly depressed, but more commonly elevated, that are distinguished from the rest by 
the semitransparency of their shell; these spots are sometimes considerably enlarged, 
and their elevation increased, so that they become prominent papilla, closely resembhng 
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 are con- 
siderably larger, and form single, double, or even triple rows between the septal bands 
(Plate V fig. 10). 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 small-pox), and distinguished from 
the preceding by the absence of any peculiarity in the texture of the sheU. That these 
and other analogous variations o-f surface-marking have no value as differential characters, 
is at once demonstrated, not merely by their gradational approximation in different mdi- 
viduals, but by the fact that they are presented in very different degrees on different 
parts of the surface of the very same shell ; the chambers of one part of the spire bemg 
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. 
148. The collection of Mr. Cuming, however, contains a group of forms which are at 
once distinguished from the rest by their general physiognomy (Plate III. figs. 11, 12), 
and which, when their characters are examined in detail, appear to be separated from 
them by well-marked differences. Their aspect is much more lustrous, and them 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 
