105 
ON THE MICROSCOPIC STRUCTURE OF SHELLS. 
withstanding that the sharpness of the divisions would seem to indicate the 
presence of membranous partitions between the cells, no trace of such can 
be discerned after decalcification, the total amount of animal matter beinc 
extremely small. This is a fact of conBiderable importance, as showing un- 
efiuiTocaily that the absence of every indicaUon of cellular structure in the 
jujimal ba^is must by no means be taken as n proof tliat tbo shell lias not 
been originally Ibrmed by the calcification of a celluliir tissue; since nothing 
unequivwai tJian the existence of cellular structure in the solid 
shell ot although, when the lime has b.-i-n dissolved away, the walla 
ol the cells cannot be made out. In this respect such shells niav be coiu- 
paml with the fossilized shells oC Pht/ia, Aviatia nm] other Marormtacece 
m which the cellular structure is as jwrfectly shown as in the recent species, 
although no trace of the animal basis remains. The internal layer of Thnida 
praents tio well^lefiued structure. 
striatn closely corresponds in structure with 
that of the ^«aAW, having a nacreous inliTior, oml a well-defined cellular 
oijta oa ihe exterior. The cells of this layer are large and Ronu-what pris- 
iMtie in form, the prisms being arranged somewhat obliquely to the surface, 
an cropping out upon it (tig. JJ3). lii SckizzMtexmn Jiititiu the structure 
® e shell more resembles that of ; the internal layer being nearly lio- 
•nogeneoiis ; whilst the exterior has an indistinct cellular structure, the cells 
*^unded, and closely packed together, 
red- P<m/iora is remarkably distinguished from all the pre- 
^ by its complete conformity with that of the Morgurilacea in all their 
t-sseDUal characters; munely, in the regular iirismotic arrangement of the cel- 
th»> exterior, the .ixcs of the prisms being perpendicular to 
tw existence of distinct membrauous partitions bc- 
anrr forming a tenacious membrane that is left after decalcification ; 
wnaM ‘^ 1 nacTCom structure of the Interior. The views of those who 
fr..m fyP® of ® distinct group receive confirmation therefore 
anvnfik of its shell, which has no counterpart amongst 
unltMl ? ,t**«t have the lobes of the mantle in great degree 
bTthfv' ■ ^ ^ relation«liip to the preceding genera, however, b indicated 
savp in ‘jf neighbouring families, 
cdlulav ^ do we meet ^vitb anything like the same definite 
turp .if u gr>nus Pt/rwsiff, in regard to the intimate struc- 
onlinai^. ^ be regarded as intenuedwte between Pafulora and the 
wtprnal 1 ®®P®cfally Ajtfitim ; for, with a nacreous interior, it lias an 
lame i<ni /T 'rith very definite boundaries, and a few 
make nni ^ V ^ scattered over its surface. I have not been able to 
that i» cellular .'tructure in the delicate membranous residuum 
tb® decaleitication of l,he shell of Lyonsia, 
Xllf. SolenacetE, 
j I 
cflhis close affinity which exists between the animals 
^ sinnify Tfw family, we find the structure of their shells bearing 
delininf 'S'o/rn ami Sohairhis the external layer has a very 
stiawbt an I** ^ *^Uicture; the cells are elongated prisms, their walls as 
P^Hel to p ^ those of Pinjia (fig. -Is)*; but their axes lie nearly 
^fiere tlioir ^ cropping out somewhat obliquely upon the exterior, 
^ *^et withTf?” 11 nuclear spots may sometimes 
The internal layer it very dense and nearly homo- 
See the Explanation of the Plates for a fuller description of this figure. 
