SUB-CLASS I TRILOBITA 609 
cheeks, which may be separate or united in front, and carry the compound sessile 
eyes when present ; cephalic appendages pediform, consisting of five pairs of limbs, 
all biramous, and functioning as ambulatory and oral organs, except the simple 
antennules, which are purely sensory. Upper lip forming a well-developed hypostoma ; 
under lip present. Somites of the thorax movable upon one another, varying im 
number from two to twenty-nine. Abdominal segments variable in number, and fused 
to form a caudal shield. All segments, thoracic and abdominal, carry a pair of 
jointed biramous limbs. All limbs have their coval elements forming gnathobases, 
which become organs of manducation on the head. Respiration integumental and by 
branchial fringes on the exopodites. Development proceeding from a protonauplius 
form, the protaspis, by the progressive addition of segments at successive moults. 
The Trilobites constitute a group of extinct marine animals, and are 
related to the stock of the higher modern Crustacea ; they are therefore to he 
considered as very primitive Crustaceans. The sub-class had its origin in pre- 
Cambrian times. Trilobite remains are very abundant in the oldest known 
fossiliferous strata, the Cambrian, where they exceed in number and diversity 
all other forms of animal life. They continue to be very plentiful in the 
Ordovician and Silurian, but decline in the Devonian, and the few last sur- 
vivors are found in the Carboniferous and Permian. Probably there have 
been more than two thousand species described, distributed among nearly two 
hundred genera. These numbers give an idea of the amount of differentiation 
and specialisation attained by Trilobites during Palaeozoic times. 
Carapace.—Trilobites were covered or protected on the dorsal side by 
hard crust or shield, which is the only portion commonly preserved. Their 
remains are therefore readily recognisable by the form and structure of this 
shield. It is divided longitudinally by two dorsal furrows, or grooves, into 
three portions or regions, and on this account the name Trilobite was first 
given. The central ‘part forms the axis of the animal, and contained the 
principal organs, as the viscera, heart, and chain of ganglia. Transversely the 
shield is divided into (a) a head portion called the cephalon; (/) a series of 
joints or segments, forming the thorax; and (¢) a tail-piece or pygidium, 
forming the abdomen. 
The test seldom exceeds one millimetre in thickness, and consists of thin 
laminae of carbonaceous and phosphatic compounds of calcium, some of which 
were originally chitinous substances. The laminae are frequently traversed 
by minute pores, which give a punctate appearance to the test, and are some- 
times large, as in Homalonotus and related forms. 
The carapace is somewhat arched or convex, generally elongate-oval in 
form, and rounded at both ends. The length is almost invariably greater 
than the width. Very often the same species shows a broad form, as well 
as a relatively larger, narrower one. The former was considered by 
Barrande as representing the female, and the latter the male individual. The 
carapace is often ornamented with spines, teeth, and knobs. These may be of 
the nature of surface ornaments, or in the case of spines, may be produced by 
growths from the genal angles, the ends of the segments of the thorax and 
pygidium, or the spiniform | extension of the byzidial termination. 
The carapace does not often terminate at the margin as a simple lamellar 
plate, but is turned under, and forms a reflexed margin, or doublure, which is 
parallel to the outer edge, but is separated from the upper sur face by a narrow, 
partially included space. This produces the hollow spines from the ends of 
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