DR. W. B. CARPENTER ON ORBITOLITES TENUISSIMA. 
555 
But after making from six to eight turns (the number varying in different individuals) 
the spire begins to open out in the horizontal plane (Plate 38, fig. 5, a) without any 
vertical enlargement, and a complete septum is formed at the next break, marking 
off the first principal chamber from the previously-formed spiral tube. This septum 
is traversed, as in Peneroplis, by a variable number (four in the specimen here 
figured) of passages, which would show themselves as pores upon its external surface ; 
but these, instead of opening into another single undivided chamber, lead into as 
many chamberlets, which are formed by the subdivision of the next principal chamber, 
1), by radial partitions, exactly as in Orbiculina. This chamber, in the individual 
here figured, is not separated by a completely-formed septum from the succeeding 
chamber, c, and the latter is undivided save by a single radial partition ; but this 
is a mere individual variation,—which is of interest, however, as showing that the 
subdivision of the chambers into chamberlets is a secondary, not a primitive formation. 
The septum which closes-in the chamber c is traversed by 13 pores, which open 
into as many chamberlets formed by the subdivision of the next principal chamber; 
the separation of these chamberlets by radial partitions being complete for about 
four-fifths of the length of the chamber (that is, of the distance between its inner and 
its outer septum), but deficient for the outer fifth, so as to leave the continuous 
gallery d, d, into which all the chamberlets open at their outer ends. This chamber, 
it will be observed, extends itself on either side at d', d', so as to enclose a portion of 
the spiroloculine <£ nucleus ;" and this extension is still more marked in the next 
chamber, whose two aim, e', e, reach the ends of the transverse diameter of the 
original spire. The septum which separates this chamber from the preceding has 
the number of its pores increased to 30 ; and these open outwards into as many 
chamberlets in the next-formed chamber. As new chambers are successively added, 
the backward extension of their aloe is carried further and further, until (in the 
individual here figured, Plate 37, fig. l) those of the ninth chamber meet at the back 
of the spiroloculine ££ nucleus,” so as to enclose it all round, and the tenth chamber 
forms a complete ring of chamberlets, whose derivation from the undivided chamber 
of the £ peneropline ’ type is made obvious by the previous transition. With each 
increase in the length of the septal plane, there is a proportionate increase in the 
number of pores by which it is traversed, the distance between them having a very 
uniform average ; and the number of these pores determines the number of chamberlets 
m the next annulus, which has thus no definite relation to that of the chamberlets in 
either of the last-formed or in the subsequently-formed annulus. The breadth of the 
zones (and, consequently, the length of their chamberlets) has a range of variation 
1 from l-180th to l-80th of an inch, its general average being l-120th inch ; so that a 
| disk having a diameter of 0'6 inch (or a radius of 0'3) would be made up of about 
forty such concentric zones. A very narrow zone is occasionally seen to intervene 
between two zones of ordinary breadth ; but, as I have always found this to originate 
