ON THE MICROSCOPIC STRUCTURE OF SHELLS. 
Ill 
growth. We here see the three layers, a, b, c, distinctly marked; and there 
is an additional layer, rf, which is the duplicature oftlio external layer that 
is formed when the animal has attained its full {growth. This section traverses 
the middle layer in the same direction with the fractured surface shown in 
%. 47, and we recognize the same clispositioo of the lauiime of which it is 
composed, the boundaries of those being indicated by the oblique lines which 
are seen crossing each other in this part of the soetiou. Uut as the laniintB 
of the other two layers are arranged at right angles to those of the middle 
layer, a section which traverses the surface* of the latter will cut the edges of 
the former; and we accordingly see the ouitT and inner layers ]ict'sentiiig an 
aspect which verj' strongly Vesemhles tiiat of the prismatic cellular structure 
of Anno, when seen in vertical section. section of the same shell, still 
perpcDdicular to its surfaces, but parallel (instead of at right angles) to the 
liuM of growth, would show this prismatic appearance in the middle layer; 
whilst the intorual and external layers would present the oblique crossed 
lines previoudy seen in the middle layer. This is c.vaetly what is shown in 
ug. 49, which jg a seetion of the shell of Conus, taken in the same rlirection 
With the preceding, i. e. transversely to the lines of growth ; the directions of 
the lamin® in Conus and Cgpresa being, as just now statiul, at right angles 
to each other. 
The idea that the Jaminte are made up of elongated cells arranged side 
hy side, and that the parallel lines of divUiyii ure iinlioutions uf tlif original 
ceihwalls, and not of the more homnlariesofcrystuls, iJorives furtlier support 
from the circumstance tliat those lines are frequently very considerably 
curved, aiitl sometimes very irregular. Thu curvature is best seen in sections 
ot the thickened lip of the full-grown shell, where a single? layer is often half 
an jiicli or mure in thickness, and is tuudu up of successive growths, marked 
by distinct bauds of colouring matter, but still in perfect continuity witli 
each other, just as in the thick shells of the large PinufO. The irregubrity 
0 the botindaries of the laniinre is best seun iu sections w lneli are taken in a 
anection nearlyto the surface of the shell. This will still cut the 
«ui^ edgewise, but in a diftcrent maoiier from a vertical section*. In 
g- w IS shown a section of Conus, which (owing to the curvature of the 
V*! Y both the internal and middle layers in a direction nearly pa- 
^ ^ surface. The edges of the laiuin® of both an: liore seen; and 
e dire^on of the one set of plnnes (a) at right anghis to the other (A) is 
J^wly shown. The principal fact, however, which section displays, is 
e Want of regularity of the boundaries of the lamina*, such as oaimut be 
structure lx wot at obcc readily acquired by every oue, bat is 
I *’^''^** attaiiiwl, it may 1« wfU to facilitate its coiiiprchousion liy a simple 
With ihr=i.”k L ^ r^prescnied by a row of hoolts arranged njwn a table 
thi* Iml .yj ^'rpo'mogt. It is obvious that a hccriun wbicli traverses them//araff''/ to 
*«lts. Bill ® scries of porollol line* formed by Its Inisirwction with the covers oi me 
wniri, w ®tber band, a sretiort winch jasses Iu the i.lann of the lca^•e* would nbow 
l*^h, \'“t ’W’old exhibit whatever surfarn the intlividaal leaves might happen to 
d* irsL u nT.i ^ *i‘''tber iuj»poi«l timt a second row of books ware HiijurTpiised upon 
b«t . 1 .' '• hwking upwards and downwards us before, 
ftese iu th5,lu f ‘he prccriUng ; then it is tibvjoui that a section which passes through 
too taut cross the lower scries in a direction parallel to thew 
ewedL nr?r i tlmsshow the surfarc of the letvea of the upiter set. but the 
"iHt swW , Lbenesih: and if a second mtion fir made at 
'itoes in tkl . I • reversal will be uroducea in the appearances which it dis- 
we two layer* respectively. 
