ype of the Lobster, are not without examples but they
become much more numerous in the succeeding Jurassic period.
Remains of insects have also been discovered.

Amongst the _Mollusca_ we have to note the disappearance, amongst
the lower groups, of many characteristic Palaeozoic types. Amongst
the _Polyzoans_, the characteristic "Lace-corals," _Fenestella,
Retepora_,[22] _Synocladia, Polypora_, &c., have become apparently
extinct. The same is true of many of the ancient types of
_Brachiopods_, and conspicuously so of the great family of the
_Productidoe_, which played such an important part in the seas
of the Carboniferous and Permian periods.

[Footnote 22: The genus _Retefora_ is really a recent one,
represented by living forms; and the so-called _Reteporoe_ of the
Palaeozoic rocks should properly receive another name (_Phyllopora_),
as being of a different nature. The name _Retepora_ has been here
retained for these old forms simply in accordance with general
usage.]

[Illustraton: Fig. 144. Triassic Lamellibranchs. a, _Daonella_
(_Halobia_) _Lommelli_; b, _Pecten Valoniensis_; c, _Myophoria
lineata_; d. _Cardium Rhoeticum_; e. _Avicula contorta_; f. _Avicula
socialis_.]

_Bivalves_ (_Lamellibranchiata_) and _Univalves_ (_Gasteropoda_)
are well represented in the marine beds of the Trias, and some of the
former are particularly characteristic either of the formation as a
whole or of minor subdivisions of it. A few of these characteristic
species are figured in the accompanying illustration (fig. 144).
Bivalve shells of the genera _Daonella_ (fig. 144, a) and _Halobia_
(_Monotis_) are very abundant, and are found in the Triassic
strata of almost all regions. These groups belong to the family
of the Pearl-oysters (_Aviculidoe_), and are singular from the
striking resemblance borne by some of their included forms to
the _Strophomenoe amongst the Lamp-shells, though, of course, no
real relation exists between the two. The little Pearl-oyster,
_Avicula socialis_ (fig. 144, f), is found throughout the greater
part of the Triassic series, and is especially abundant in the
Muschelkalk. The genus _Myophoria_ (fig. 144, c), belonging
to the _Trigoniadoe_, and related therefore to the Permian
_Schizodus_, is characteristically Triassic, many species of the
genus being known in deposits of this age. Lastly, the so-called
"Rhaetic" or "Koessen" beds are characterised by the occurrence
in them of the Scallop, _Pecten Valoni