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of many species; and these remarkable Molluscs must have 
swarmed in the seas of the Middle Lias. The history of their 
internal structure is curious and unique; and, beyond the 
belief that they formed the internal skeleton of a naked 
decapod, we know nothing of their ancestry or their relations 
in time. The chambered shells are well represented in the 
Lias. The Nautilus group has descended from the older 
formations, and retains its typical form in the shell of the 
Nautilus striatus, of which we have many fine specimens in 
our collections. It is Ammonites, however, which afford us so 
many subjects for study; and the number of species of this 
group—which extended from the Trias to the close of the 
Chalk era—is so great (amounting to 2000 species) that we 
may now consider them a distinct order, composed of several 
families, and each containing several genera. The Trias 
Ammonites all belong to specialised forms, which are limited 
to that formation. Amongst the earliest tenants of the Lias 
sea these beautiful chambered shells played an important part, 
and appeared under forms of a novel character. The great 
group of the Areitites, with their prominent keel and bisulcate 
siphonal area; to these succeeded the groups Aguceras, Lyto- 
ceras, Harpoceras, &c.; each of these being characterised by 
special points in their anatomy, by which they are readily 
distinguished by experts in Paleontology. There is no pretence 
for saying that the newer forms of any of these groups have 
been evolved from pre-existing Ammonites; at all events, said 
the Doctor, I have never been able to trace out the anatomical 
character of such genetic relations. 
The Doctor next passed on to the Radiate animals, of which 
he specially selected for illustration the Hchinodermata. In 
this class, said he, we have a most marvellous piece of anato- 
mical structure in the mechanism of their skeleton, which 
affords ample means for comparing one form with another, as 
a test of the theory of the evolution of forms of the Lias from 
those of the preceding Trias age. When we compare the Lily 
Encrinite of the Muschelkalk with the Pentacrinide of the Lias, 
we see at a glance how different the one form is from the other, 
