112 
REPORT-1847. 
attributed to crystalline action, but is perfectly accordant with the suppcsition 
of organic structure. A similar want of regularity is shown in tin* section 
of the inner layer of Patella, represented in fig. 51. This has traversed lie 
laminae not quite across their edges, but somewhat obliquely, so that adif- 
ference of disposition is evident in the alterunUj lamina*, which (it will U 
remembered) are directed obliquely across one anotlier. This difference L' 
brought strongly into view whiin a thin section is examined by polarit^i 
light; for in certain positions of the prisms every alternate baud is darkened, 
whilst the intervening bands remain bright; and when one of the pri'^iiHa 
turned half round, the bands wliich \v«Te at tirst bright bi*cojne dark, wd 
viee versa. The effect is very Ijcautiful when a plate of selenite is piaetd 
behind the .‘»ection ; for the complementary colours are then exhibited by th' 
alternating bands, and the revolution of the prism causes an exchange« 
these. 
93. Such is the general structure of the shells of tire Gasteropod Molltuct 
especially those of the order Peclmihvanclnnfa. I'hc most interesting ei* 
amples of departure from the arrangement now describetl will be pri’sontl)’ 
brought under notice. I shnll first, however, allude to certain indication of 
the changes which the shell midergoCvS during the progress of its gro"^ 
which are of considerable intercat. It was remarked by Mr. Gray in tfs* 
paper already referred to, that, ho long as the shell is in a growing 
the outer layer projects beyond the niiddb! one, ami the middle hp't 
beyond the interior one ; but that when the aniuial has attained its ad'iH 
state, the middle and inner layers are brought up to the margin, to foro^ 
thickened lip. In young sheila the outer layer often projects far Wyoud tbe 
others. “ Upon examining," says Mr. Howerbunk (loc. “ a young fipr* 
cimen of AmpuUariu faseiuta, I found that at the extreme margin, and to 
about onc-eighlh ol‘ mi inch backward, there was but one stratum of 
the plates of which were iwrallel to the lines of growth. At about one* 
eighth of an inch inwards, a new stratum of cells, with plates at right anp'f 
!f iT commenced, and rapidly inereoaed in thiebas as '• 
receded from the lip • until, at about half a revolution of a whorl backwori. 
It had attained a thickness equivalent to about one-fifth of the entire 
stance of that part of the shell; and at one revolution backwards it 
^ thickness to the layer above it." It is obvious 0 “ 
aridition can be made to the outer stratum after the subjacent laver 
V upon its ej'ternal surface, as in 
the thickness of the middle stratum aifi rthe fw* 
Ih?oUn.?i 1 this last-lomied internal layer i.iaTl» 
tmckonecl by successive deposits to any extent; and this appear U* 
the thickness of the internal layer at5ui« 
that iftli Mm*"'^** ^ considerably greater proporti^ 
at ot the middle and c.\ternal layers than it <loes nearer the iiiargin- " l-l”* 
8 t^X“ “tbe relative degrees of thickness of thi« 
The ^ found the following vari^ 
forL u h^fh ' ‘n Plates near the mouth of a young >/*• 
?u7nber or '"a? growth, may be represented by 
ber At half tlui revolution of a whorl backAvarii thr 
v-aa rr„d by 20. whila thr now atrato.n boaea* U ...Lurrd 1S;1^ 
the lower 'to upper stratum measored 1*-^ 
a like m' youoS" grew in substance. U]* 
the s mtrn .1’ ‘"I! “P'«“on “f Sulimus ohlongus, Uie 
strata near the bp were as follow, :_outer 10, ccS^ba lb tosr 17 i ^ 
