ox THE MICBOSCOnC STRUCTURE OF SHELLS. 127 
bounded hj nearly straight edges. I have not been able to obtain specimona 
enough of the several species of Pentacrinife to follow out tliis inquiry as it 
(Ipscrres; but I may remark, that, so far as my knowledge extends, the vari- 
ation of itilenial structure in different jiarta of the samo stem is very trifling, 
--much Itas, in fact, than nught he inferred from the surfaco-markings of 
the joints,—as delineated by Goldfuw and Dr. Bucklnnd. 
121. Whilst in the Pmtaerinna there is an entire absencis of any appear¬ 
ance of cnncentric arrangement, we meet with this in the stems oi Apux-riniles, 
Adinocrinites, and other round-stemmed Encrinites, which do not show any 
treco of the medullary rays of the PcDtacrinile. The diameters of their clo- 
niPJitary structure are precisely conformable to tho genera! type, and do not 
present any speciality thot can serve for the Utscriminadon of species or 
^era. The structure of the plates of the body, hi these and other CVhioi- 
is exactly the same as that of tho pieeos of the testa of Echinun. 
}^' remains for us to notice the ordvrn Ilolothuruula and Sipun- 
In whicJi, as is well-known, the calcareous skeleton of the other 
Erkinodermatn is reduced to its most rudimentary condition, never forming 
a complete aud connected framework, but only showing itself in detached 
I'Koes, the disposition of which is extremely variable. In the llolothuria 
discovered a few yearn since on the Cornish coast by Mr. C. W. I’eacb, there 
are fivesolid calcareous plates around the mouth, in which the characteristic 
Mtieularion is well seen. Each of the teiitacula also has a small calcareous 
ducat its extremity, which is a sort of rude sketch of that of the ambulacral 
"uckers of the Echinus. I am not aware that any trace of a calcareous 
skeleton has been found in cither of the genora of tho order Sijmw'ufacefB ; 
ot looking to tho extraordinary unity in the plan of organization, which is 
s ioffn in the elementary structure of the skeleton throughout tho entire 
cass of Bchinodernuitai I cannot doubt that should hucIi rudiments be dis¬ 
covered, they would present a similar character. 
XVIII. Crustacea. 
123. la regard to tho microscopic striieture of tho hard envelopes of Crus- 
I can asyct only speak from examination of tho common Crab and 
ih f which this examination has disclosed arc so curious, 
. to delay the publication of llumi until I have ox- 
w ed the inquiry to members of other divisions of the class; more espe- 
ihp ^ paper by M. LavalJe on the ‘ Microscopic Structure of 
JO ’,"^*1 Decap^ Crustacea’ (Atinalcs dca Sciences Naturollcs, .Tune 
it j,'t • appt’^r to me to he very imperfectly appreciated. I think 
“-iMt to myself to quote, in the present Report, tho duBcription which I 
gve of tills structure in ray first communication to the British Association 
rinco, and published in tlu- Annals of Nutimil History for De- 
lavcN “ The cMvtdope of the Crab and Lolrstor consists of three 
in lim’ ® calcareous layer, the surface of which is raised up 
the t clcv.itions ; upon this lies a layer of pigment-cclis to which 
p’ ^ due; and exlermilly is a structureless horny epidermis. The 
valleys or iotcrvuls between tlte [wpillary elevations 
cover tlic latter. The epidermis, which 
*0 th*»^ lies flat upon tho whoh?, adhering to tho tops of the papillae ; 
a Biift 11 ** closely cxauiined from the outside, it is wicn to have 
y speckled appeoKiijcc, the red ground given by the piginenbcells 
The cfll**' of the papillae which rise up through it. 
composed of a substance exactly analogous to ivorj*, 
8 very transparent and apparently homogeneous, when cut into very 
