File:The Quarterly journal of the Geological Society of London (13206139854).jpg

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PROCEEDINGS OF THE GEOLOGICAL SOCIETY. .Mar. 10,
the neck-segment in all Trilobites ; the latter segment in our genus
is armed with a spine.
rr^
Fig. 1. Trinucleus ornatus.
2. Do. magnified fringe.
3. Hypothetical development of the fringe.
4. Fringe of T.fimbriatus,
The body-rings are six, very flat, and capable only of bending or
doubling, not of rolling up as in Calymene — a character of considerable
importance, inasmuch as it has relation to the connecting membranes
and articular surfaces of the crust : species capable of rolling into a
ball requiring large articular surfaces on the segments to enable them
to move freely on each other ; and a fulcrum, as centre of motion, on
the side pieces of each segment, is generally placed about one-third of
the distance from the axis : — in Trinucleus the rudimentary fulcrum
is quite at the extremity of the segment, and apparently useless.
The tail, generally smooth, has a deflected border, but not, as far
as observed, a striated internal fold, so common in higher forms of
Trilobites.
The peculiar perforate border is the most interesting part of these
animals, and I propose to examine it critically.
The puncta are almost always arranged in radiating rows, three,
four, or more holes in each row, and these being at equal distances
they form concentric lines. In T. granulatus^ two of these rows are
separated by a furrow from the rest ; in T. seticornis, three are distinct
from the remaining two or three, by the front rows being sunk in a
deep concentric furrow. Other modifications take place : in T. jimhria-
tus, the two front rows are turned downwards ; lastly, in T. ornatus —
for by that name we must call T. Caractaci — the dots occur most
frequently in quincunx order — i. e. the radiant rows appear zigzag,
and not direct (fig. 2) : this appearance is due to the great obliquity
of the rays (2, a a). I wish to call attention to this, because I con-
sider it enables us to understand the nature of the enigmatical puncta.
If we suppose a head furnished with a produced membranous margin