116 
REPORT—1847. 
sent as in the more massive shclU. The whole thickness of the shell is 
commonly so small, however, that it is impossible to recognise them in asec* 
tion perpendicular to its surface ; but they are at once distinguished from 
each other, in a section that traverses them all somewhat obliquely, by the 
different arrangement of their elongated cells, the long diametew of whid 
lie in each layer at right angles to tiiose of the next. 
101. Notwithstaiuling the remarkable general conformity in the structufs 
of the shell throughout the extensive group of PeftinUtmiKhiata, CfttM 
variations present themselves in particular families or genera. Thus in annuli 
species o^Natica,\ have found the cells of the middle layer more distiwl 
and pavement-liko than usual (fig. 58). In Turbo, TVocAwj and 
the inner layer of the shell is usually nacreous ; and I have been abluwdi* 
stinguish in the nacre of these shells the same minutely relluIararrangeincDi 
which I have described as jiresenting itself very distinedy in Hahotif, h 
the black middle layer of Turbo pica the cellular arrangement is very di* 
stinct, and the arraugemont of the colouring matter dispersed through the 
calcareous contents of the shell is very beautiful, as shown in fig. 59. 1“ 
the operculum of a large Turbo, also, a very beautiful cellular structure he 
presented itself ; a pi>rtiou <if this (In w hich tlie section has traversed ih® 
lusitorm cells obliquely) is represented in fig. fJO. 
102. It will be obvious from the foregoing details, that the microscopic 
examination of the shells of Gasferojioi/a has not been as fertile in regard M 
the variety of Its results, as a similar exuminution of Bivalve didk Ii« 
proved; and that we cannot look to it for much assistance, either in heljticg 
to hx the systematic position of shells whose other charnctcj^ mar leave il 
ofshJli the genus or family to which a rainiite fragme"' 
stratPff *1 ttio investigation has most unequirocaily de’ioun- 
Ga.<Jcropo(la, like those of LameUilravchiaU!.^^ 
conforSr\ FT and that this arrangement iscverfwl.c« 
rchZrrjr, . ndHin of all 
“Ltafniauer’." “» 
XVI, Cephniftpodii. 
on\S'whofc*innr^*^'* **‘^**^ existing testaceous Cephalopoda “ 
shelU oT h^TnT l^i'-alves, than to that of the Unidr. 
cScI sL' Fh of perpendicular 
qi>ite indistinguUhablf. TheshelM 
their differenre nf I t' Presence of Ooo layers is obvious from 
ternal one is f “j ”*^'*''*^* being nacreous, whilst the »• 
external layer of the sbeF f *"^escence. A thin section oft^e 
face, show 3 ^tl.a» 5 i /Jornpila^s, taken iiarallel to the suf 
those of the Iiaercm. *F bearing a strong reseiuhhuJK 
ffoaaula tho "a.uoT,!,hv ^ I- tbe‘l>in 
tile outer laver of a"* structure can he easily distinguisltflt 
inten, “ ay?r at V,*'"* ^ ‘■“a able to recognise in 
“yer at all comparable to the nacreous structure irluch mates up> 
