oose-bill" Lamp-shell (_Lingula anatina_), and
thus presents us with another example of an extremely long-lived
type. The _Lingulelloe_ and their successors; the _Linguloe_, are
singular in possessing a shell which is of a horny texture, and
contains but a small proportion of calcareous matter. In the Upper
Cambrian Rocks, the _Lingulelloe_ become much more abundant, the
broad satchel-shaped species known as _L. Davisii_ (fig. 32,
e) being so abundant that one of the great divisions of the
Cambrian is termed the "Lingula Flags." Here, also, we meet for
the first time with examples of the genus Orthis (fig. 32, f,
k, l) a characteristic Palaeozoic type of the Brachiopods, which
is destined to undergo a vast extension in later ages.

[Illustration: Fig 32.--Cambrian Fossils: a, _Protospongia
fenestrata_, Menevian Group; b, _Arenicolites didymus_, Longmynd
Group; c, _Lingulella ferruginea_, Longmynd and Menevian, enlarged;
d, _Hymenocaris vermicauda_, Lingula Flags; e, _Lingulella Davisii_,
Lingula Flags; f, _Orthis lenticularis_, Lingula Flags; g, _Theca
Davidii_, Tremadoc Slates; h, _Modiolopsis Solvensis_, Tremadoc
Slates; i, _Obolela sagittalis_, interior of valve, Menevian;
j, Exterior of the same; k, _Orthis Hicksii_, Menevian; l,
Cast of the same; m, _Olenus micrurus_, Lingula Flags. (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