32 Messrs. W. K. Parker and T. R. Jones on the 



Planorbis, D'Orb.) the pseudopodial foramina are extremely small 

 compared with those of larger and more inflated varieties (such 

 as R. vesicularis, Lam.). 



O. vesicularis seldom preserves its simple single disk of cells ; 

 for not only do the umbilical cells increase in number, and be- 

 come perfect in form, but the upper series have one or more 

 superimposed layers of similar annuli, the primary septa of 

 which are immediately adapted to the earlier septal rings*. 

 These upper or additional layers may or may not extend over 

 the whole area of the first system of rings, being sometimes 

 confined to the centre and heaped up ; but sometimes they ex- 

 tend all over, and even beyond, the primary disk. As this 

 growth becomes more perfect in regularity and in the number 

 of its layers, it leads us to the next variety. 



6. Accompanying No. 5 are others, differing in shape ; some 

 high, like a sugar-loaf, and others subhemispherical. Dr. Car- 

 penter has pointed oat to us that in these forms (some of which 

 are -| inch in diameter) not only is the regularity of arrangement 

 in the overlying annuli well marked, but a vertical section 

 presents several tiers of cells, separated laterally by radial septa, 

 which pass upwards and outwards from the primary cells to the 

 periphery. At the same time, the umbilical cells strive, as it 

 were, to overtake the cyclical series in their growth. They in- 

 crease in potency, taking on a regularity of arrangement almost 

 equal to that of the upper cells ; and the inferior surface of the 

 shell becomes flat, and even convex. The umbilical cells have 

 now an annular arrangement, and, like the others, are placed in 

 tiers, but with shorter radii ; for they are still fewer than on the 

 other face, and hence the shell is unsymmetrically biconvex. 

 The primaiy cells are necessarily subcentral, lying nearest to the 

 umbilical face. 



The upper surface now loses almost all trace of the annular 

 structure, from the increasing importance of the polygonal 

 arrangement of the secondary cell-walls. The polygons in 

 No. 5 were elongate somewhat in the direction of the annuli ; 

 but now they have become more regular throughout. The upper 

 set of chambers now grows mutually with the umbilical ; the 

 two sets being welded together at the edge and growing together. 

 This variety may be termed 0. congesta ; it has passed from the 

 Patelline to the Orbitoline form. 



7. We have also from the same coral-mud numerous spherical 

 specimens, differing from the foregoing in shape, but not gene* 

 rally larger (about -g inch). Their structure is absolutely similar; 



* Our attention has been lately drawn to this form of growth by Dr. 

 Carpenter, who has been engaged in researches on some of the larger forms 

 of this group. 



