hell "winged," or with the lateral angles prolonged to such
an extent as to have earned for them the popular name of
"fossil-butterflies." The closely-allied _Spirifera disjunda_
occurs in Britain, France, Spain, Belgium, Germany, Russia, and
China. The family of the _Productidoe_ commenced to exist in the
Upper Silurian, in the genus _Chonetes_, and we shall hereafter
find it culminating in the Carboniferous in many forms of the great
genus _Producta_[17] itself. In the Devonian period, there is an
intermediate state of things, the genus _Chonetes_ being continued
in new and varied types, and the Carboniferous _Produdoe_ being
represented by many forms of the allied group _Productella_.
Amongst other well-known Devonian Brachiopods may be mentioned
the two long-lived and persistent types _Atrypa reticularis_
(fig. 97) and _Strophomena rhomboidalis_ (fig. 98). The former
of these commences in the Upper Silurian, but is more abundantly
developed in the Devonian, having a geographical range that is
nothing less than world-wide; whilst the latter commences in the
Lower Silurian, and, with an almost equally cosmopolitan range,
survives into the Carboniferous period.

[Footnote 17: The name of this genus is often written _Productus_,
just as _Spirifera_ is often given in the masculine gender as
_Spirifer_ (the name originally given to it). The masculine
termination to these names is, however, grammatically incorrect,
as the feminine noun _cochlea_ (shell) is in these cases
_understood_.]

[Illustration: Fig. 98.--_Strophomena rhomboidalis_. Lower Silurian,
Upper Silurian, and Devonian of Europe and America.]

[Illustration: Fig. 99.--Different views of _Platyceras dumosum_,
of the natural size. Devonian, Canada. (Original.)]

The Bivalves (_Lamellibranchiata_) of the Devonian call for no
special comment, the genera _Pterinea_ and _Megalodon_ being,
perhaps, the most noticeable. The Univalves (_Gasteropods_), also,
need not be discussed in detail, though many interesting forms
of this group are known. The type most abundantly represented,
especially in America, is _Platyceras_ (fig. 99), comprising thin,
wide-mouthed shells, probably most nearly allied to the existing
"Bonnet-limpets," and sometimes attaining very considerable
dimensions. We may also note the continuance of the genus
_Euomphalus_, with its discoidal spiral shell. Amongst the
_Heteropods_, the survival of _Bellerophon_ is to be recorded;
and in the "Wi