Arthropoda. — The  only  known  Cambrian  Arthropods  are  the
Crustacea,  and  of  these  much  the  most  abundant  group  is  that
of  the  Trilohita,  which  are  altogether  confined  to  the  Palaeozoic
rocks,  and  are  by  far  the  most  important  of  Cambrian  fossDs.    It

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PALAEOZOIC  ERA


is  only  within  recent  years  that  the  systematic  position  of  the
Trilobites  has  been  established  through  the  fortunate  discovery
of  specimens  with  their  appendages  attached  (see  PL  VII,  Figs,  i,
I  a),  Trilobites  have  a  more  or  less  distinctly  three-lobed  body,  at
one  end  of  which  is  the  head-shield,  usually  with  a  pair  of  fixed
compound  eyes;  at  the  other  end  is  the  tail-shield,  and  between  the
two  shields  is  a  ringed  or  jointed  body  made  up  of  a  variable
number  of  movable  segments.    The  Trilobites  display  an  ex-


Plate  II. —  Cambrian  Trilobites

Fig.  X,  /fo/mta  brdggert  Wale,  x  ^  L.  C.    2,  OUnellus  ikontpsoni  Hall,  x  V,  L.  C
(Walcott)

traordinary  variety  in  form  and  size,  in  the  proportions  of  the
head-  and  tail-shields,  in  the  number  of  free  segments,  and  in  the
development  of  spines.  Already  in  the  Cambrian  this  wealth  of
forms  is  notable,  though  far  less  so  than  it  became  in  the  Or-
dovician.  As  compared  with  those  of  later  times,  the  Cam-
brian Trilobites  are  marked  by  the  (usually)  very  small  size  of
the  tail-shield,  the  large  number  of  free  segments,  and  their  in-
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Plate  III.  — Cambrian  Crustacea

Fig.  X,  Piyckoparia  ktngt  Meek,  x  ^4  M.  C,  a,  P.  antiquitata  Salter,  X  %,M.  C.  3,
Crtpicephalus  iexanus  Shumard,  x  y^,  M.  C.  4,  Mesonaeis  vermontana  Wale,  x  \<^^  L.
