DR. W. B. CARPENTER ON ORBITOLITES TENtflSSIMA. 



(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 O. complanata 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 0. 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. marginalis nourishes 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 protoplasmic 

 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 OrTjitolites tenuissima. 



Fig. 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, enlarged to 64 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 



