108 
REPORT— 1847. 
plan of construction that seems common to by far the greater number of 
them; and any considerable departures from this are uncommon. Tlte 
small proportion of animal matter contained in most of these shells is a retj 
marked feature in their character, and it serves to render other features in- 
distinct; since the residuum left after the removal of the calcareous matters 
usually so imperfect, as to give no clue whatever to the explanation of the 
appearances shown by scctiunsi, &c. In fact, as Mr. Hatchett observed loo; 
since, the porceI{a7iOtts siiolls do not yield any distinct animal residuum 
whatever after the action of nrids; and the presence of animal matter ii 
them is made more evident by the blackening of the substance of tlieslfdb 
when heated, than by any ()tljor method of treatment. Neverthdejs tlw 
structure of these shells is by uo means homogeneous, but always exIiWU 
indications, more or less clear, of an original organic arrangemeuL This 
arrangement, however, has been regarded by Mr. Gray (Phi). Trans. IK38I 
as the result of crysttUUne action; and it was not surprising that it shouldbf 
thus regarded, when tho real nature of shell was so cmuplctelymisundpretood, 
considered as a mere eamthtimt of calc.’ireous matter, the parridfs 
or which were held together by a sort of aiiirnal glue. I have alreaiiv (sbuwii 
that no such explanation is applicable to the shells of AVma and its allia 
in which the elementary crystalline forms were suppo-sed by .Mr. Gray m b« 
ot the prismatic order; since the>e shells, instead of coiitaming Hip W rf®" 
portion of animal matter (as stated by Mr. Gray of the siiells that present 
the appearance of a crystalline fracture), have tho animal bash; 
and It IS perfectly ulivious that the appearance of prismatic crystaliizatinr* is 
entirely due to the moxtliUng —.so to speak—of the calcareous matter witiiin 
he long prismatic cells of their shell-membrane. And there appears to «afi 
ample refuson lor extending a similar exj)lanaf.ion to the shells in wliieb the 
rhoraboidal crysfallization seems to present itself; for although uo distinct 
cellular membrane can be obtained by their dccaJcificatiim, we have found 
other shells, W'hose origin is unquestionably cellular; 
ana i think it will be obvious, from tlie details into which I sJiall presentij 
enter, that the structural characters of these slielU arc decidedly 
ami not dependent upon crystalline action. But I may again reiuari tlut 
w f ^ olumentary parts of shell as tti- 
g rom crjstallmc action, the molecular arrangement or aggregation I'f 
S tri'-sT 
tiireVtht*'!h!n * given by 3Ir. Gray of the gtmoral stnif 
conti^L s n?.- statement of appearances wbicM 
G ’s evnhn^f' ^ as far as it goes, although 1 do not accord with Mt* 
Unival^ H fiagments of most of the spind 
Pie^ef i fm oi' th?fractured .sides of the cubic 
on S Z lilTw the centre; an 
bv a flat broken sidi*H, shelving and internal edges, comiected 
&rent <l‘»hrences of surface being prSduced by the 
laverrthuf ‘ i ‘1 different layers. Each of the thr« 
iTside \''-T " '■'’"'loosed of very thin lamin®,placed side 
la^ce Wlmn ^*‘“^hiics8 ul the plate, and perpendicular to its sur- 
with obscum^ M- minutely examined, they will be found marked 
broke.i direction of which they separate, wbeD 
ceedins limin*^ nanow rhombic er 3 * 8 tals. The lines of cleavage in the sue- 
pSs SnS ® " contrary direction.s tliat when^two of these 
ted are placed under the microscope, the lines of cleavage appear 
