102 
REPORT— 1847 * 
in the translucent intervening spaces some vestiges of cell-boundaries may 
be discerned. 
VIII, Cardiacea:. 
74. Of the structure of the shell in this family there is little to be said, as 
it is vei^- nearly the same with that of rise preceding. In various species of 
Cardium-whic^y I have exaniiiicd, I hare found a considerable amount of 
tubular structure in the externa! layer, whilst in Isorardia there is scarcely 
a vestige of it; thus bearing out the general sfalcntent that the presence of 
tlie tubular structure has a relatiun to the costations, foliations, or other 
sculpturing of the external surface. There is usually but little organic struc¬ 
ture to be discerned in the internal layer; ami the only characteristic of the 
external layer, independently of the tubular structure, is its corrugated aspect. 
These negative characters, howfver, are not without their value, for they 
show that the structure of the shell in this family has nothing whatever in 
common with that of the remarkable Plvurorhi/nchuif hibemicus, formerly 
termed Card'm/n hihernicum, with wliicli some of the species of Carditat 
correspond so (doaely in external form. ’J'he structure of this remarkable 
shell has been described in uiy loruier lleport (§ 34). 
IX. Coiichacea:. 
75. Still less of organic structure is presented by the shells of this family, 
some of which are the liardcsl that arc to be met with amongst the Bivalves, 
approaching the porcellauous Univalves both in density and in the almost 
eutire absence of any trace, of animal matter. Tbe internal and external 
layers here dififer in little ol.se than their relative transluccncy, the latter being 
often rendered senii-opuke by the deposition in their substance of a Qtiauliiy 
of calcareous matter in a iion-crysUilIinc condition. In some species, a few 
scattered tubuli are to be met with ; but these arc seldom so definite either 
m size, number, or arnmgemmit, as to bo at all cliaractcristic of the shells 
which present them. In several members of this family, liowevcr, I have 
detected such traces of cellular structure, as imlicate that the original mode 
of formation of the shell is not dificreiit from that of other Bivalves. Those 
traces are chiefly to be nicf. «ith (as in other instances) beneath the epider- 
mis, where such exists; and they are especially evident in the translucent 
shells of various species of Cj/cha. In some of these, too, there is a more 
detimte animal residuum than in most other Conchacea, and traces of cells 
may be discerned in it. 
76. The shells of this family closely resemble those of the preceding in the 
solidity of their texture; but they usually display clearer indications of or- 
game structure. Both in the external and internal layers we frequentlT meet 
with apprarances winch can scarcoly be due to any other cause than a pns- 
matic Mllnlar structure, although the prisms are far leas regular in form than 
m XMG Margantacea., and ore not so definitely divided by aninial metnhrawe. 
1 lie direction of the prism.s too is diffci-ent from that which they have in the 
group just named, for instead of having their axes perpendicular to the sor- 
lace, so that their extremities present ihcnuidves when that surface 
mined, they he nearly parallel to the surface, or rather in a slightly oblique 
manner, so as to crop out upon it. Their ends are usually ratberpointed than 
termed/,/siforro cells, the direcrian of these 
evflifn'n '“yere, however, is not the same; for whilst in the 
nim their axes usually lie in lines radiating from tbe binge to the 
bin of each valve, they have a direction transveree to this in the internal 
