GEXUS OPERCULINA: EXTERNAL CHAEACTERS. 17 



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

 third whorl, if it has not previously done so, acd is accompanied with a corresponding 

 augmentation of its breadth, so that the septal plane becomes narrowed and elongated, 

 as is seen in comparing fig. 4 with fig. 3. That no basis for specific distinction is 

 afforded by the most marked differences in the form of the spire, as shown in the pro- 

 portions of the septal plane (A, B, c, D), will be made evident by the examination of such 

 a transverse section as is shown in Fig. V. (p. 21) ; in which it will be observed that 

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

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



145. Another marked feature of difference among the specimens of this collection, is 

 the depression or elevation of the central region relatively to the peripheral. In what 

 I have assumed 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 

 such forms as are represented in Plate III. figs. 1 and 2, the umbilical region is rather 

 depressed than elevated ; and this depression is often observed in older specimens whose 

 early growth has taken place on this type. But there is a group of specimens, of which 

 three sticcessive ages are represented in Plate III. figs. 3, 4, 5, 8, that have the whole 

 central region so exceedingly prominent as to form a cone, whose apex is marked either 

 by one large tubercle, or by a cluster of smaller ones ; and this conformation gives so 

 peculiar a physiognomy to the shells which present it, that few systematists would hesi- 

 tate in placing them apart as specifically different from the rest. On a careful compari- 

 son 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 tubercle, as in fig. 7, through 

 those in which the region generally is slightly elevated, as in fig. 8, to those in which it 

 presents the most marked projection, as in figs. 3-5. We shall presently find (*f[ 153) 

 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. 



146. A fourth very obvious character of differentiation among the individuals of this 

 collection, consists in the presence or absence of tubercles on the septal bands. In what 

 I have described as the typical form (Plate III. fig. 7), there are no considerable pro- 

 minences 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 occupied 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 only 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 extends to the second, 

 and even to the third, as is well shown in fig. 6. The specimens whose central region is 



JfDCCCLIX. D 



