96 
REPORT— 1847- 
cificd membrane, I have several times found them disposed to split, ia tbe 
manner represented in fig. 5. The fissures have always taken place exacdg 
in the line of the striations ; showing just that tendency to separation at 
these parts of the prisms, which might be expected if the supposed coalescence 
had imperfectly taken place. In some instances it will he seen that the 
layers thus separated still contain two, three, or four of the single origiml 
layers •, but at a, a will be seen the edges of single layers, which have bewnie 
detached for a part of their lengUt from those with which they were pre¬ 
viously incorporated ; and at by h are portions of similar layers that are w 
twisted upon themselves, as to exhibit their surface as if it had been ex¬ 
posed by a section in the contrary direction. This surface presents the 
division into irregularly-hexagonal cells, which Is characteristic of the struc¬ 
ture; and we have thus an excessively thin layer, separating itself naturally, 
but showing us precisely the appearances of a similar layer artificially nb- 
teitied. No trace of a vessel is here to be seen, the divisions of the cells 
being formed by the deposit of animal matter between them in unusual 
thickness. 
bo. JSayade<E.'—A vertical section of the shell of Unio ocddetiS', in whict 
thq prismatic cellular structure is remarkably thick, has afibrdtd me the op¬ 
portunity of ascertaining that the pnsuiatic cells, when brought into view io 
a longitudinal direction (fig. p), have exactly the same characters as those 
of the Margaritact’te ; and also uf showing the nmniu-r in which the shell b 
enlarged by the addition of new layers. The ordinary statement—that the 
sliells of the Concliiferous Acuphala are progressively enlarged by the depo- 
Bitioii ot new layers, each of which u in apposition with Uie interaai surtaoc 
oi the preceding, and nt the same time extends beyond it— doe not expreES 
e whole truth; for it takes no account of the fact that most shells are 
composed of two layers of very different texture, and it does not specify 
.7 . n entire surface of the mantle 
i-pfo,- t ®''*y Produced. A 
Mieience to fig. 8 will, I think, clearly show the mode in which the operation 
represents a section of the margin of one of the 
perpendicularly to its am-Le, aucl exhibits 
or nZ.T'f composed; the upper one being the outer 
tioJ• Tn ! .* 1 . traversed io^dtudinally bythesec- 
ttuvirln iw® the internal or nacreous laver. The latter is 
sire fornntLn'* n*r*i?*^* j'bos, a a\b b\ andee', which evidently Indicate succm- 
fthe left haiH** ^ those lines may be traced from the margin 
£hat at ‘’f rennty of the figure) towards the umbo, in such a manner. 
Xhe f therefore, in the oldest if 
aL of nn ! tJie number of enlargements it has undeigone, as the 
ite .ten. P.?Hi n r®'" ** by the number of concentric rii^ 
tact with tW m ‘I • "k ‘ '“f "acreous lamina is in mmediate con* 
is norf rn^ ‘he newthus produced 
porlv coiistUnfrl ^■•f-substatice, but of that only which pro- 
the mamh^ i ^-tructure hegoml 
caDniiitf^ii nfl ‘ ’ i'find one layer of it over- 
tioS the luicH of junction between two distinct form^ 
and k annpnri T ^ ‘’gRb I'nwever, the arraiigeraeut is more complicated; 
ful d mE « even wlfen the shell has attained i* 
entire “hell bpL 7 to bo formed one after the other, the 
^hell being i,„ed, as before, with a thin layer of nacre, whilst the Up 
