(Alter
Salter, Hicks, and Davidson.)]
Of the higher groups of the _Mollusca_ the record is as yet but
scanty. In the Lower Cambrian, we have but the thin, fragile,
dagger-shaped shells of the free-swimming oceanic Molluscs or
"Winged-snails" (_Pteropoda_), of which the most characteristic
is the genus _Theca_ (fig. 32, g). In the Upper Cambrian, in
addition to these, we have a few Univalves (_Gasteropoda_), and,
thanks to the researches of Dr Hicks, quite a small assemblage
of Bivalves (_Lamellibranchiata_), though these are mostly of no
great dimensions (fig. 32, h). Of the chambered _Cephalopoda_
(Cuttle-fishes and their allies), we have but few traces; and these
wholly confined to the higher beds of the formation. We meet,
however, with examples of the wonderful genus _Orthoceras_, with
its straight, partitioned shell, which we shall find in an immense
variety of forms in the Silurian rocks. Lastly, it is worthy of
note that the lowest of all the groups of the _Mollusca_--namely,
that of the Sea-mats, Sea-mosses, and Lace-corals (_Polyzoa_)--is
only doubtfully known to have any representatives in the Cambrian,
though undergoing a large and varied development in the Silurian
deposits.
[Illustration: Fig. 33.--Fragment of _Dictyonema sociale_,
considerably enlarged, showing the horny branches, with their
connecting cross-bars, and with a row of cells on each side.
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