DR. W. B. CARPENTER ON ORBITOLITES TENUISSIMA. 557 



the former, whilst in the multiplication of its nuclei it obviously tends towards the 

 latter. 



The growth of this beautiful organism doubtless takes place after the completion of 

 the first annulus, in the manner described in my former Memoir ( 35). The sarcodic 

 body, when enlarged by the nutriment it has appropriated, will project itself through 

 the marginal pores, in quantity sufficient to form, by the coalescence of its separate 

 protrusions, a continuous belt of sarcode ; and in the substance of this a set of 

 radial calcareous partitions will be deposited, commencing between the pores of the 

 margin of the previous shell-ring, while two horizontal lamellae are formed on the 

 superficial planes, to constitute, as it were, the floor and ceiling of the new circle of 

 chamber] ets. These horizontal lamellae overlap but very slightly the margin of the 

 previous annulus (Plate 37, fig. 3, a, a) ; and their adhesion to it is generally so weak 

 that the annul! readily come apart. As the subdivision of the annulus into 

 chamberlets does not extend to its outer portion, a passage is left (seen in vertical 

 section at b, b, b, fig. 3), which is occupied by a continuous ring of sarcode, as shown in 

 Plate 38, fig. 1 ; and from this ring proceed the stolons which pass outwards through 

 the pores of the septum that closes it in. 



The homogeneousness of the protoplasmic substance by which the entire cavitary 

 system is occupied, is shown (as in the types formerly described) by the completeness 

 with which the effects of injuries are repaired, and the plan of the original fabric 

 restored (see Phil. Trans., 1856, Plate VIII., figs. 4-9). Not only is the loss of any 

 part of the disk repaired by the formation of a new and continuous annulus along the 

 broken as well as the unbroken margin, so that the next and all succeeding zones 

 follow the new contour (as shown in Plate 37, fig. 6) ; but a new and entire annulus 

 of chamberlets may form itself around the whole circumference of a mere marginal 

 fragment (tig. 7), by the enclosure of which in a subsequent succession of annuli, the 

 discoidal form characteristic of the type is completely and characteristically restored. 

 Owing to the transparence of these attenuated disks, I have been able to assure 

 myself that every part of the margin of this fragment, whether broken or unbroken, 

 peripheral, central, or lateral, has contributed to the formation of the first new 

 complete annulus, by which the foundation was laid of the subsequent regular series 

 of concentric zones ; thus clearly indicating that a sarcodic extension took place from 

 every chamberlet laid open by the fracture, as well as from the normal pores of the 

 last-formed septal plane, and that these extensions coalesced to form a continuous ring, 

 as in the formation of the ordinary succession of concentric annuli. 



This perfect reproduction of a form of peculiar regularity, in a type of animal organisa- 

 tion so low that its body-substance does not show any advance upon the primitive proto- 

 plasmic condition, is, in itself, a matter of great interest. But the interest is much 

 enhanced by the consideration that this organism begins life, and forms its first shelly 

 envelope, upon a plan altogether different ; exchanging this for its later mode of 

 growth, by a transition so rapid as to manifest the almost sudden attainment of a 



MDCCCLXXXIII. 4 C 



