ON THE MICBOSOOPIC STRUCTURE OP SHELLS. 
123 
continuity towards the centre, until they abut against tlie pillars of the layer 
next within. 
IIS. Little doubt can, I think, be entertained that the successive layers 
are, as in exogenous trees, tlie result of successive formations or acts of 
growth; their deposition taking jilace in the substance of the iiieiiibrane 
which envelopes exterior of the .‘<pines, in the same maimer os the addi¬ 
tions to (he edges of the several plates of the shell are inivde by the calcifi¬ 
cation of a portion of the membrane which dips down between tbeni. At 
what inicn'ids these additions are made, wo have at present no means of de¬ 
termining. The manner however in which they modify thts form of the spine, 
as shown by sections in botli directions, is often very curious. In fig. 73 is 
diown a longitudinal section of part of one of the large club-shaped spiues of 
HeAiRMatra/M passing through about two-thirds <»f its length, and allow¬ 
ing the mode in which the successive growtlis are disposed. It w til be seen 
that the spine, as first develojied, terminated at d. The next layer cumpletcly 
invested this, but added much more to the length of the Hjiiue than to its dia¬ 
meter; its termination is seen at «. The successive layers, the tcniiiiiacions 
of four of which are seen A andi, mark out the succession of sizes which 
the spitiD possessed after each addition; the accrctiou being always so much 
greater in the longitudinal than in the transverse direction, as to preserve 
the original proportions of the spine tliroiighoiit. The consequence is, that 
a section taken across tiu* line h v, near the base of the spine, will exhilrit all 
the layers of which the spine is made up; each layer being narrow, an«l the 
central medulla small. A section taken across the lino h I, at about the 
puddle of the length, will not traverse the original spine, nor will it bring 
mto view anything older than llio fourth layer, wltln’n which the central ine- 
d'llla will be of larger size 5 and the tciUil uumUT of concentric layers which 
It presents will be three ftiwer than tliat shown in the preceding section, 
although its area will be greater. Lastly, a section taken across the spine 
nwr its extremity will bring into view, although of larger diameter than 
either of the preceding, only the last one or two layers, of which the inner 
oijc embraces a large area of loose mndullary reticulation. 
114. One of the most curious featuri’S dificloscil by transverse sections of 
wch spines, is the very iinifonn manner in which the radiating vertical plates 
01 each layer are connected with the pillars of the preceding. This is parti¬ 
cularly well seen io fig. 70. We do not sec a fresh scries interposed in the 
cxternul layers, the miniber being M;ldoui increased. I have now and then, 
owever, observed one or two additional pillarH in the outer layers; but the 
I' . ®'^'ch should pmw inwards frriin these towards the ccutre soon become 
ID the general reticulation. 
. 115. Another interesting feature in the structure of tbeHC spines consists 
*? taken to repair an injury. This ot least is the interpretation I 
appearance winch I have sm'cral times mot with, and of 
oich hg. 65 will give an idea. It will be observutl that the continuity of 
c row of pillnrs forming the second layer from the centre i# interrupted at 
hi reticulation at that part has a confused appearance, as if it had 
wtw beaten in by a dntt ou the outside of the spine. Over this portion, and 
^nilmg for some litUo way on either side of it, we observe a thin layer, 
» with lU own set of pillars, which, not extending around the ciiiirc spine, 
to have been formed to repair the injury of the subjacent portion. 
^DD Uie whole is covered in by the next regular layer, which extends um- 
ormly around the whole npine.' It would hence appear that the membrane 
vtriDg the spine has not only the power of depositing new layers of the 
careous network at certain regular periodsi, but that it can also, like the 
