100 
REPORT—1847- 
IV. Tridacnaceee. 
70. Of the shells of this family there is but very little to say, since neither 
in Tridacm nor in Hippoims is there any distinct organic structure. Here 
anti there obscure traces may be discerned, indicative of their cellular origin; 
but the principal departures from perfect homogeneity are occasioned by what 
I have termed on a former occasion the wmtgatcd structure (see § 18, and 
fig. 43 of first Report). Of the real nature of that structure, as elucidated 
by my subsequent observations, I shall have more to say immediately. 
V. ChamaccxB. 
71. In the species of the genus Cliama tre again encounter a re. 
ticulated tubular structure, closely rcsenibliiig that which has been already 
described in Anomia e.p/npjnum and Lima srabm. (See §§ 30, SI, and fig. 
40 of first Report.) This is chiefly found in the outer layer of the shell mid 
around the bases of the foliations. In those species, liowirver, in which thtse 
projections arc least developed, the tubular structura scarcely presents itself: 
and as I find thn same to hr the case in flio genus iJnm, I am inclined to think 
that this structure lias usually some relation to the formation of these curious 
appendages. In one portion of a section of the middle layer of a non-foliatcd 
Chania, I have mot witli acellular arrangement (fig. 13, it) eo distinct as to 
be uniuistakcable, mitwitlistanding that it does not prc-Hcnt itself in tlir iiu'in- 
bmuoas residuum. Hy a change in the diroclion of the layers of the slu'll 
iclativciy to the plam* of tho section, the latter pusses (at h) more obliquely 
across the partitions of the colls; and by a still further change, it travims 
(at c) the divisions in a longitudinal direction. Now thoM' db-isious are such 
as 1 have before suj>pos(.'d to indicate mere corrugations in the baiemenl 
membrane; but niim the appearance here presented by the transverse sec* 
tion, and from otlier indications, I am now ilisposml to regard them as formed 
oMoog prismatic cells bcuriug a general resemblance to those of Pinna, hut 
much less regular lu form. As the lung prUms of Pinva are made up by 
the coalescence of a pile of llattomid cella (§ 6‘J), go would it appear that 
these IcK perfect structures have u similar origin, since delicate Sxiffl luay 
cross them frausverscly or obliquely (sec § 78, 
fr,... f -1 ^ “^'’6 R^f’cady stated my resison for separating 
Is .i.r ??’* •''’ ‘i’” /xtcriml layer of highly-organized priMuatit' 
the T- ‘'Imractcrs) iU approximation to 
a^ruTl Lfn structure is typical. Of ihe structure of aido- 
And 1 rp 'rr bad an opportunity of examining die shell. 
^tmcturlTnl?^ ’ of a good specimen in which the testaceous 
the niiniitP il fl ^ comlition to speak positively us to 
bv sZl r f eenos Diccras. wl.ich has Zen supposed 
J^otW Ll l«-‘Vween the C/iamaa ie and the 
of Ih^vP^t. in 6nch specimens as I have examined, 
of forer^rpotr <^“=«-ac.orWc of that group (S 32 
VI. Triffonaettt. 
indude?U7^fI“‘^V**^ '■'f- Trigoftia and Castalia, the two genera 
dVawuletF *if'iV*i” is such as to bring us back towards the 
beS^; [n*- 'vhilst the interior ot the shell is composed of a thick layer of 
tracuui Ihel is covered with a hornv perios- 
» there intervenes between them a distinct and w'ell.developed cellular 
