571 
DR. W. B. CARPENTER ON ORBITOLITES TENUISSIMA. 
(as it were) of the “spiroloculine” coil into a single Milioline turn of greater thickness; 
but the Orbiculine or second spiral stage is fully retained. 
In 0. duplex, the abbreviated Milioline centre is still retained, but the succeeding 
Orbiculine spiral is almost entirely dropped out, quickly giving place to the cyclical 
plan. 
And in the typical 0. complancita the Milioline centre is immediately surrounded by 
a complete annulus, so that nothing remains of the original spire save the one turn of 
the circumambient segment. 
So, in the passage from the “ simple ” to the “ complex ” type, we have a remarkable 
anticipatory step in O. duplex, which can scarcely be supposed itself to derive any 
advantage from the substitution of a double for a single row of communications 
between the annuli, since 0 . mdrginalis flourishes equally well with its single row ; 
but which forms, so to speak, a stepping-stone to a higher grade. 
Everything in this history, then, shows a well-marked progressive tendency along 
a definite line towards a highly specialised type of structure in the Calcareous fabric; 
and this without any corresponding departure from the original homogeneity of the 
Animal body which forms that fabric. And as being, so far as I know, altogether 
unique in these peculiarities, I venture to offer this study of a humble proto'plasmic 
organism, brought up from an ocean-depth of nearly two miles, to the consideration 
of those who believe with Sir James Paget, that “ the highest laws of our [biological] 
science are expressed in the simplest terms in the lives of the lowest orders of 
Creation.” 
Explanations of Plates. 
PLATE 37. 
Structure of Calcareous Disk of Orbitolites tenuissima. 
Pig. 1. Surface of young disk, showing its eccentric spiroloculine “ nucleus,” giving 
origin to successive zones of orbiculine chamberlets, which gradually increase 
in breadth with the opening-out of the spire, until they extend completely 
round the nucleus; after which the successive additions are made on the 
cyclical plan, as concentric annuli. Magnified 25 diameters. 
Fig. 2. A portion of three peripheral annuli, eidarged to G4 diameters, and partially 
laid open by the removal of the superficial lamella, so as to show the two 
annular septa, aa, bb, the chamberlets, c, separated by radial partitions, and 
the annular gallery, d, into which all the chamberlets open at their peripheral 
extremities. 
Fig. 3. Vertical section of three annuli of the disk, taken in the radial direction, so as 
to traverse the chamberlets lengthways ; a, a, junctions of two annuli, with 
