GENUS HETEROSTEGINA. 291 



divide it both from the adjacent chamberlets of the same row, and from the chamberlets 

 against which it abuts at its extremities, are all double. The latter (forming the septa 

 between the principal chambers) are strengthened by the interposition of an additional deposit 

 of shell-substance, forming an "intermediate skeleton ;" but this is scarcely traceable in the 

 former. There are distinct indications of a canal-system resembling in its general distribution 

 that which is more fully developed in C//clocl>/peus (T 498); and these are most obvious 

 (Fig. XLVI, h, h) in the thickened marginal band, which resembles the "marginal cord" of 

 Operculiua (^ 444), save in its much inferior development. 



490. By examining a vertical section, such as is shown in Fig. XLVIl, it is unmistakeably 

 shown that the spire is perfectly symmetrical, and that each convolution not only surrounds 

 but completely invests its predecessor. The investing whorl does not, in the younger part 

 of the spire, come into immediate contact with the two surfaces of that which it includes, 

 but is separated from it by alar prolongations of the chambers and of their septa, very much 



Fig. XLVII. 



Vertical section of a youug Heierostegina, showing its symmetrical plau of growtli : a, a, openings between the 



chamberlets of successive rows. 



as in ordinary Numm/iIi/itB (^ 462), these alar prolongations showing little or no subdivision 

 into chamberlets. But between the later whorls there are no such interspaces, and the 

 successive laminte come into absolute continuity with one another ; both the tubuli and the 

 cones of non-tubular substance being continued from each into the one external to it. From 

 the time that the rapid thinning-away and opening-out of the spire commences, the investment 

 of the previously formed whorls seems to discontinue ; the spiral lamina of the last whorl 

 being merely applied to the external margin of that of the penultimate (as in OjxrcuUi/a, % 442), 

 instead of being continued over its surface. 



491. JJjfiJiities. — It is obvious from what has preceded, that Heterostc/jina bears precisely 

 the same close relation to Operculina that Orhicid'ma bears to PeneropUs ; the subdivision of 

 the principal chambers into a row of chamberlets, by partitions extending transversely from 

 each septum to the next, being the essential character of difference in each case. The 

 relation of analogy between Heterosteghia and Orbiculiiia is extremely striking ; since in each 

 type the earlier convolutions invest those which precede them, whilst the later extend 

 themselves peripherally instead of centrally, and the mouth of the spire or " apertural plane " 

 widens on each side so as to pass round (it may be) the whole circumference, and tlius to 



