250 FAMILY NUMMULINIDA. 



earlier period of growth is evinced by their smaller number of convolutions and of chambers, 

 and beinf^ often seen, moreover, in the earlier whorls of those which afterwards present the 

 greatest flattening, may safely, I think, be regarded as partly a character of age. It will be 

 recollected that the young of the flattened Fencropl'iH often resembles BendrUina in the com- 

 parative turgidity of the spire (1[ 130) ; but I do not find that Operculum ever continues long- 

 to increase upon such a plan ; for the compression af the spire always shows itself in the third 

 whorl, if it has not previously done so, and is accompanied with a corresponding augmenta- 

 tian of its breadth, so tliat the septal plane becomeh' narrowed in Fig. XL. Such a section 

 makes it obvious that no basis for specific distinction can be afforded by the most marked 

 differences in the form of the spire, as shown in the proportions of the septal plane ; since 

 differences equal to those presented by the most compressed and the most turgid forms of the 

 spire are there exhibited by the successive convolutions of one and the same individual. 



435. Another marked feature of difference to be noted among individual OpercuIuiah^Q 

 depression or elevation of the central region relatively to the peripheral. In what I have as- 

 sumed as the typical form, the central region presents the same general level with the rest, though 

 the umbilicus itself is often marked by a prominent tubercle. In young specimens with a turgid 

 spire, however, the umbilical region is rather depressed than elevated ; and tiiis depression 

 is often observed in older specimens whose early growth has taken place on this type. But a 

 form is not uncommon, in which the whole central region is so exceedingly prominent as to 

 form a cone whose apex is marked either by one large tubercle or by a cluster of smaller 

 ones ; find this conformation gives so peculiar a physiognomy to the shells which present it, that 

 few systematists would hesitate in placing them apart as specifically different from the rest. On 

 a careful comparison of a large number of individuals, however, it becomes apparent, that this 

 difference, like the preceding, is gradational; every degree of prominence being traceable from 

 the individuals which have the umbilicus marked only by a tuljcrclc, through those in which 

 the region generally is slightly elevated, to those in which it presents the most marked pro- 

 minence. We shall presently find (^441) that this difference depends mainly on the degree in 

 which the investing layer, prolonged from the later convolutions over the surface of the earlier, 

 is separated from that surface by the extension of the alar prolongations of the chambers of 

 the investing whorls ; as to which point there is a most remarkable diversity, not only among 

 different individuals, but between the several convolutions of the same individual. 



436. A third very obvious character of differentiation among the individuals of this col- 

 lection, consists in the presence or absence of tubercles on the septal bands. In wliat I have 

 described as the typical form, there are no considerable prominences over the greater part of 

 the surface ; the septal bands are generally smooth and continuous ; and it is only in the 

 central region that we observe any departure from this uniformity, the umbilicus being occu- 

 pied by a small tubercle, and the smooth septal bands being replaced in the first whorl by 

 moniliform rows of little tubercles. In other instances, however, we find not onl}' the central 

 tubercle, but the rows of tubercles marking the septa, much larger and more prominent ; and 

 this marking-out of the septal bands by elevated tubercles is not limited to the first whorl, but 

 may extend to the second, and even to the third. The specimens whose central region is very 

 prominent usually show this feature in the most decided manner ; but it is occasionally pre- 



