ON THE MICROSCOPIC STRUCTURE OF SHELLS. 
95 
of species under the two heads of “ perforated ” and “ not perforated,” which 
I ^ve in my former report (§ 42); Ter. cmrctuki and Ter. suhrotunda beiiiff 
perforated, whilst Ter. acuta is not perforated. 
Placunid<E.-~As the “prismatic cellular structure” appeared to be 
peculiarly characteristio of the group of shells with the lobes of the mantle 
completely divided—presenting itself most typically in the MargurUacecc, and 
in a subordinate degree in the Ostracets on the one hand and the Unionida 
on the other—I could not rendiiy account for its apparently complete 
absence m the shell* of tliis family. I hare since ascertained, however, that 
a vwy thin but distinct layer of it may sometimes he traced on the exterior 
ol Anomia ephippium (fig. 7) ; and i am disimsed from analogy to believe 
that It K constantly formed in the first instance, but is subsequently more or 
leas completely worn away. Since my former Report, I have also examined a 
^i^^QiPkainanomia', and have found that whilst its geimral texture 
f«e®hle3 that of its coogenei-s, there is one part of the shell—that 
which bears the muscular impression and surrounds the passage for the so- 
called bony attachment—which has amore solid and less latninatiHl character, 
and 18 made up of a distinct cellular structure. Hie “bony attachment” of 
Anmui and Pkcu7ianomia difiers entirely in structure from the shell to 
which It belongs; I have not been able, however, to satisfy myself fully in 
regard to tlie plan of its formation, and I therefore refrain from now at- 
raptiug to describe it. I may state, however, that it has nothing m common 
texture, save the large proportion of animal matter 
winch It contains. 
stated, I have always been on the 
fh ot the prismatic celiular structure on the exterior of 
e t ells oi tills group also ; but I had not succeeded in distinctly tracing 
jnem at the time of my I'onner Report. 1 was disposed to account, for this 
js'jpposing that the first-foruieil or external portions of the shelly laj’ers are 
Bim!* ^ perhaps in consequence of the comparatively active move¬ 
ments ot these animals. My expectation Ims been in some degree justified by 
iscovery of a tliiu hut beautifully distinct layer of this substuoce on the 
tho^ t represcnled in fig. 6 . X wouhl suggest it to 
ih,. **^'^*^ opportunity of prosecuting the inquiry, to examine 
tnit^ fi- 1 Rectoris, which may be usually viewed by trans- 
''■•fl’out any ])reparation. 
—Jn ilcscribiug the prismatic cellular structure, as it is 
e iH the shell of Pinna oud its allies, J drew attention 11) to the 
!^ii 1 ?* upon the membranous walU'of the pris- 
ihesf, ****'* prisms which they enclose, when 
I «i«^ structure arc obtained in ii separate form. And 
thkam ^*^‘^1** (§§ 12, IS) for not asuciiting to the inlerprcmtion of 
ufli-red by Mr. Rowerbank, who rcganls tlicse strife a* indi- 
•imole tli‘ i tubes; iiut for considering them as produced by a 
of flat cell-incjiibramuat tlio points whore sacccssive layers 
prisQiati^” II *'*'“'^“***‘ in pile*, so as to make up the long 
Since , 1 ^ which characterize the fully-formed shell substance, 
(luestint? rejioafed and careful examinatioas into this 
Af o ^ ^ present wliat I beliovo to be demonstrative evl- 
vaiioii, ^ cwectnctt of the view which I Imd taken. Aionngst the 
r«markala*^f '^l’>ch 1 have examined, tlie I*i/tna rwliii U most 
linltiM quantity of animal matter interfiosod Iwtween the 
trans^rsA cell*, and also for the clcuniess and strength of the 
mar mgs on their walls. In making vertical sections of its decal- 
