_), an operculate genus which has given
palaeontologists much trouble in its diagnosis, for it has been
regarded as a pelecypod (hippurite) and a brachiopod. The tabulate
corals were represented by _Favosites_, _Michelinia_, _Pleurodictyum_,
_Fistulipora_, _Pachypora_ and others. _Heliolites_ and _Plasmopora_
represent the alcyonarians. Stromatoporoids were important reef
builders. A well-known fossil is _Receptaculites_, a genus to which it
has been difficult to assign a definite place; it has been thought to
be a sponge, it may be a calcareous alga, or a curious representative
of the foraminifera.

In the Devonian period the brachiopods reached the climax of their
development: they compose three-quarters of the known fauna, and more
than 1100 species have been described. Changes were taking place from
the beginning of the period in the relative importance of genera;
several Silurian forms dropped out, and new types were coming in. A
noticeable feature was the development of broad-winged shells in the
genus _Spirifer_, other spiriferids were _Ambocoelia_, _Uncites_,
_Verneuilia_. Orthids and pentamerids were waning in importance, while
the productids (_Productella_, _Chonetes_, _Strophalosia_) were
increasing. The strophomenids were still flourishing, represented by
the genera _Leptaena_, _Stropheodonta_, _Kayserella_, and others. The
ancient _Lingula_, along with _Crania_ and _Orbiculoidea_, occur among
the inarticulate forms. Another long-lived and wide-ranging species is
_Atrypa reticularis_. The athyrids were very numerous (_Athyris_,
_Retzia_, _Merista_, _Meristella_, _Kayserina_, &c.); and the
rhynchonellids were well represented by _Pugnax_, _Hypothyris_, and
several other genera. The important group of terebratulids appears in
this system; amongst them _Stringocephalus_ is an eminently
characteristic Devonian brachiopod; others are _Dielasma_,
_Cryptonella_, _Rensselaeria_ and _Oriskania_.

The pelecypod molluscs were represented by _Pterinea_, abundant in the
lower members along with other large-winged forms, and by
_Cucullella_, _Buchiola_ and _Curtonotus_ in the upper members of the
system. Other genera are _Actinodesma_, _Cardiola_, _Nucula_,
_Megalodon_, _Aviculopecten_, &c. Gasteropods were becoming more
important, but the simple capulid forms prevailed: _Platyceras_
(_Capulus_), _Straparollus_, _Pleurotomaria_, _Murchisonia_,
_Ma