Of these the last (fig. 32, i) is highly
characteristic of these ancient deposits; whilst _Discina_ is
one of those remarkable persistent types which, commencing at
this early period, has continued to be represented by varying
forms through all the intervening geological formations up to the
present day. _Lingulella_ (fig. 32, c), again, is closely allied
to the existing "Goose-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.
Pages:
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162