ON THE MICROSCOPIC 8TIIUCTCRE OP SHELLS. 
109 
to cross each other at right angles, whilst tiiose of the alternate laniin® fol- 
low the same direction. The laiuin® of the outer and Inner plates are 
always directed from the apex of the cone of which tlie shell is formed 
towards its mouth ; in the spiral shells they consequently follow the direction 
of the spire. Oc the contrary, the laminaj of the plate situated between the 
Ofher two, form concentric rings around the cone, parallel with its base, and 
c«ss at right angles those of the inner and outer layer. A gocid illustration 
of this structure may be obtained by cxamiiiing with a pocket-glass the 
Iractured edge of a Cone, Olive, or other spiral shell, in which the ertremifies 
w the Uminaj of the outer and inner plates and the sides of those of the 
wntral layer, or the converse, will be observed, accortling to the direction of 
the fracture, the extremities of the lainiiise showing the angles of the erj’- 
swls, while their sides, when closely examined, will often exhibit the ciwstal- 
hne flakes." (Phil. Trans. 18SS, p. 790.) 
9^ The account of this structure given by Mr. Howerbank in liis paper 
on thu ..tructure of Shells, to which I have alrcaily referred (Microscopical 
InuisactioDf, vol. i.), corresponds with my own olwervations in almost tivery 
parucvilar; and as T have been obliged on otlier points of this iiiquiiy to ex- 
pn?« my dissent from his views, I have the more plea-snre in employing his 
a^age for the dtseription of the ajipearances now under consideration : — 
w prodding to investigate ilie structurn of adult slioUs, I selected for 
Maimaauou thirteen specimens belonging to the eight following genera:— 
Coau.?, Pyrukt, Cassis, Aj^itullaria, Oliva, VoliUa, ami Tiidimvs. 
« " ole of these agree in the nature of their stnictu rc in the most perfect 
anner, with the exception of a mode of arrangement which I shall pn^sently 
min I ^ univalve shells, made by the lapidary and exa- 
iliaf^ ^ transmittod light, afforded ho little infaniiutian, that 1 had imme- 
11114 aid of the Lieberkulni; and upon exatnliiiog in this 
til.. u^' 1 *^? fractured at right angles to the outer and inner planes of 
sneii, and eith^ parallel or at riglit angles to the lines gwwHi, I ob- 
their distinct and satisfaetoiy view of the organic structures and 
^ particularize the results of the exa- 
deser'°»” "'hole thirteen species operated on, but at once select for 
matt ' ■ T niauriliana, which, from tho great quantity of colouring 
Ihe ^ with its structures, is tlie best adapted for examinution by 
fmchiiw) ^ .‘T'ted. Upon submitting to exaiuinatimi a aurfaco of this shell 
thrtv r *^'®*'* angles to the lines of growth, it was found tu consist of 
in tht Uniform in the nature of their structure, but alternating 
platii* '^ii*P'^“ition. Each stratimi is formed of innumerable 
diSiiJ clongate<l, prismatic, cellular structure ; each phite con- 
laia^ta parallel to ejich other. Thcsi- plates of cel- 
‘‘^desof contrary directions, so that each 
•hole-nf beneath it nearly at right angles and the 
with disposed, that the axes of the cells form angles of about 
Wce ret undi-r surfaces of the shell, and present the appear- 
*" %■ st-'O” ■w^ith a iwiwer of 280 lim-ar, the portion 
‘‘fWmaH layer of the Hheil. 'J’lic planes of thene plates 
at ^■®"“la*' structure are always as nearly as possible cither parallel 
triable i,,"i of growth, and tin: mode of arrangement is in- 
wthfr aim ^ * separate stratum, and always opposcil to that ol the stratum 
he parall.Tf ^ ‘f ‘he plates of cells in the cenlrel stratum 
«n*ta apJ.. • , growth in the shell, those of the outer and inner 
prisQiatip .4 to them. When we view the stratum of plates of 
cells, which are disposed at right angles to those I have just de- 
