many as two thousand species are known. Genera with small metasomatic
carapace, consisting of three to six fused segments distinctly marked
though not separated by soft membrane, are _Harpes_, _Paradoxides_ and
_Triarthrus_ (fig. 34). In _Calymene_, _Homalonotus_ and _Phacops_
(fig. 38) from six to sixteen segments are clearly marked by ridges
and grooves in the metasomatic tagma, whilst in _Illaenus_ the shield
so formed is large but no somites are marked out on its surface. In
this genus ten free somites (mesosoma) occur between the prosomatic
and metasomatic carapaces. _Asaphus_ and _Megalaspis_ (fig. 39) are
similarly constituted. In _Agnostus_ (fig. 40) the anterior and
posterior carapaces constitute almost the entire body, the two
carapaces being connected by a mid-region of only two free somites. It
has been held that the forms with a small number of somites marked in
the posterior carapace and numerous free somites between the anterior
and posterior carapace, must be considered as anterior to those in
which a great number of posterior somites are traceable in the
metasomatic carapace, and that those in which the traces of distinct
somites in the posterior or metasomatic carapace are most completely
absent must be regarded as derived from those in which somites are
well marked in the posterior carapace and similar in appearance to
the free somites. The genus _Agnostus_, which belongs to the last
category, occurs abundantly in Cambrian strata and is one of the
earliest forms known. This would lead to the supposition that the
great development of metasomatic carapace is a primitive and not a
late character, were it not for the fact that _Paradoxides_ and
_Atops_, with an inconspicuous telsonic carapace and numerous free
somites, are also Cambrian in age, the latter indeed anterior in
horizon to _Agnostus_.

[Illustration: FIG. 34.--Restoration of _Triarthrus Becki_, Green, as
determined by Beecher from specimens obtained from the Utica Slates
(Ordovician), New York. A, dorsal; B, ventral surface. In the latter
the single pair of antennae springing up from each side of the
camerostome or hypostome or upper lip-lobe are seen. Four pairs of
appendages besides these are seen to belong to the cephalic tergum.
All the appendages are pediform and bi-ramose; all have a prominent
gnathobase, and in all the exopodite carries a comb-like series of
sec