DR. W. B. CARPENTER ON ORBITOLITES TENTJISSIMA. 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 Lhe next principal chamber, 

 b, 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 alee, ei ', 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 alse is carried further and further, until (in the 

 individual here figured, Plate 37, fig. 1) 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 

 in 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 

 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 



