ondary processes.

(After Beecher, from Zittel.) ]

On the other hand, it may well be doubted whether the pygidial or
posterior carapace is primarily due to a fusion of the tergites of
somites which were previously movable and well developed. The
posterior carapace of the Trilobites and of _Limulus_ is probably
enough in origin a telsonic carapace--that is to say, is the tergum of
the last segment of the body which carries the anus. From the front of
this region new segments are produced in the first instance, and are
added during growth to the existing series. This telson may enlarge,
it may possibly even become internally and sternally developed as
partially separate somites, and the tergum may remain without trace of
somite formation, or, as appears to be the case in _Limulus_, the
telson gives rise to a few well-marked somites (mesosoma and two
others) and then enlarges without further trace of segmentation,
whilst the chitinous integument which develops in increasing thickness
on the terga as growth advances welds together the unsegmented telson
and the somites in front of it, which were previously marked by
separate tergal thickenings. It must always be remembered that we are
liable (especially in the case of fossilized integuments) to attach an
unwarranted interpretation to the mere discontinuity or continuity of
the thickened plates of chitinous cuticle on the back of an Arthropod.
These plates may fuse, and yet the somites to which they belong may
remain distinct, and each have its pair of appendages well developed.
On the other hand, an unusually large tergal plate, whether terminal
or in the series, is not always due to fusion of the dorsal plates of
once-separate somites, but is often a case of growth and enlargement
of a single somite without formation of any trace of a new somite. For
the literature of Trilobites see (22*).

[Illustration: FIG. 35.--_Triarthrus Becki_, Green. a, Restored
thoracic limbs in transverse section of the animal; b, section across
a posterior somite; c, section across one of the sub-terminal somites.

(After Beecher.)]

[Illustration: FIG. 36.--_Triarthrus Becki_, Green. Dorsal view of
second thoracic leg with and without setae. en, Inner ramus; ex, Outer
ramus.

(After Beecher.)]

[Illustration: FIG. 37.--_Deiphon Forbesii_, Barr. One of the
Cheiruridae. Silurian Bohemia.

(From Zittel's Palae