106 
REPORT— 1847- 
geneous; but distinct traces of cells are sometimes to be met with even in 
this (fig. 42). In no part of the shells of these two genera, however, doe* 
the decalcified membrane distinctly exhibit a cellular structure.—The case 
is very different, however, with Solemya australis, which has a remarkabljr 
firm celiulo-membranous residuum (fig. 40). The external layer of this sbdl 
is composed «if long prismatic oell-s whose axes lie nearly parallel to its sur¬ 
face (tig. .^9), whilst its intenia! layer also exhibit-s io many parts a cellolsr 
arrangement that strongly calls to mind the prismatic cellular structure of 
Pima and its allies, not merely io the form and collocation of the cells, aad 
the detinitenoss of their inembranuus ImumlarieH, hut also in the existence 
of many of the peculiar dark colls referrad to in nty former report (§ 14). 
This cellular arrangement, however, is not equally distinct in every part w 
the shell. In figs. and .48 is shown a very intonating gradation, frutn the 
definite boundaries which present themselves in some portions of thointernal 
as well as in the external layer, to the fainter and more interrupted marking* 
which still serve to indicate the original cellular composition of parts that 
have lost nearly all their membranous structure.—-The structure of the shell 
in Glycimeris closely corre-sponds with that which has been descrihed in 
Solen. 
XIV. T\tbicol(e. 
85. The reinainiug genera of the Lamcllibranchiatc Bivalves do not pre¬ 
sent any speei.-il peculiarity iti the structure of their shells, which for the 
most part exhibit few traces of organic arrangement. We shall stop, how¬ 
ever, to deaeribe the shelly tube of SqAaria. tjiffa-nfea, a niicmseopic oxfinii- 
nation of which brings Into view several points of much interest. This tube, 
formed (like that ot tlio Teredo) from tli/r.Hurfuco of the respiratory siphon*! 
several feet in length, and front half on inch to an inch iu thickue**- 
When broken across, thn fractured surfarc. has a very crystalline aspect, such 
as might easily be supposed to be given by prisma radiating from its centre 
to its circumference; corresponding in this respect precisely with tbu 
presented by the calcareous external coue of tho Beletunite. Tbifi appear* 
ajjce is however deceptive ; for microscopic examination shows that the sup* 
posed prisms have by no means the slr.iight angular borders which they 
would possess if they worn the products of crystalline action, but have iite- 
gular sinuous edges, resulting from the manner in which they werestfitst 
produced by the action of tho (»lcigerou9 cells. The shelly'tube is every¬ 
where composed of at least tvvo layers. The outer one i» thin, and contaiii* 
a good deal of uncrystallized calcareous mntter, giving it an opake-tvhite 
chalky appearance; it is easily detached from the inner layer, and i*®* 
tremety fragile; its apiM*arance, when traversed by a section ns nearly p»* 
^ j suriaec as iU cylindrical fonn will permit, is shown in fix’- 
ihe divisions there shown are so Mull defined, tliat notwithstanding Ui«f 
large size (which renders them almost visible to the naked eye), I ara dis¬ 
posed to regaril them a.s the boundaries of cells, formed jii-rljaps by the cca- 
lescence ot sraaUvr om s. Another view of this outer layer, as seen in a 
tion transverse to the axis of the tube, is soon in lig. 46 «; its cells are here cut 
iongitudinally, ami are seen to have somewhat of a prismatic form, lying 
parallel to each other, side by side. 
86. It is to the inner layer that the thickness of the shelly tube is chiefly 
Uue. I’rom the appearances presented by transverse sections, I am inclined 
f f^ ^ addition is followed in the extension of 
uie tube as in the enlargeineut of tlie valves. It is to be remembered, that, 
*e animal penetrates deeper and deeper into the sand, the respiratory 
