an eighteen and may be
grouped only as head (prosoma) and trunk or may be further
differentiated. A telsonic tergal shield of greater or less size is
always present, which may be imperfectly divided into well-marked but
immovable tergites indicating incompletely differentiated somites. The
single pair of palpiform appendages in front of the mouth has been
found in one instance to be antenniform, whilst the numerous post-oral
appendages in the same genus were bi-ramose. The position of the
genital apertures is not known. Compound lateral eyes present; median
eyes wanting. The body and head have the two pleural regions of each
somite flattened and expanded on either side of the true gut-holding
body-axis. Hence the name of the sub-class signifying tri-lobed, a
condition realized also in the Xiphosurous Arachnids. The members of
this group, whilst resembling the lower Crustacea (as all lower groups
of a branching genealogical tree must do), differ from them
essentially in that the head exhibits only one prosthomere (in
addition to the eye-bearing prosthomere) with palpiform appendages (as
in all Arachnida) instead of two. The Anomomeristic Arachnida form a
single sub-class, of which only imperfect fossil remains are known.

Sub-class (of the Anomomeristica). TRILOBITAE.--The single sub-class
Trilobitae constitutes the grade Anomomeristica. It has been variously
divided into orders by a number of writers. The greater or less
evolution and specialization of the metasomatic carapace appears to be
the most important basis for classification--but this has not been
made use of in the latest attempts at drawing up a system of the
Trilobites. The form of the middle and lateral regions of the
prosomatic shield has been used, and an excessive importance attached
to the demarcation of certain areas in that structure. Sutures are
stated to mark off some of these pieces, but in the proper sense of
that term as applied to the skeletal structures of the Vertebrata, no
sutures exist in the chitinous cuticle of Arthropoda. That any partial
fusion of originally distinct chitinous plates takes place in the
cephalic shield of Trilobites, comparable to the partial fusion of
bony pieces by suture in Vertebrata, is a suggestion contrary to fact.

The Trilobites are known only as fossils, mostly Silurian and
prae-Silurian; a few are found in Carboniferous and Permian strata. As