﻿.New 
  Species 
  of 
  Fossils. 
  95 
  

  

  cave, 
  posterior 
  lateral 
  angles 
  produced 
  into 
  long 
  nattish 
  

   spines, 
  composed 
  of 
  the 
  extended 
  marginal 
  rim 
  and 
  a 
  broad 
  

   extension 
  of 
  the 
  lateral 
  halves 
  of 
  the 
  occipital 
  segment, 
  giving 
  

   a 
  slight 
  concavity 
  to 
  the 
  outer 
  surface, 
  slightly 
  incurving 
  and 
  

   tapering 
  to 
  acute 
  points, 
  terminating 
  opposite 
  the 
  pygidium 
  

   at 
  its 
  posterior 
  third. 
  Glabella 
  prominent, 
  oblong, 
  convex, 
  

   broadest 
  at 
  the 
  center, 
  anteriorly 
  rounded 
  and 
  somewhat 
  

   abruptly 
  curved 
  downward, 
  separated 
  from 
  the 
  cheeks 
  by 
  

   shallow 
  dorsal 
  furrows. 
  Occipital 
  segment 
  broad, 
  well- 
  

   defined, 
  thickened 
  at 
  the 
  posterior 
  margin, 
  separated 
  from 
  the 
  

   glabella 
  by 
  a 
  broad 
  shallow 
  furrow, 
  which, 
  narrowing, 
  extends 
  

   back 
  of 
  the 
  eyes, 
  two-thirds 
  the 
  distance 
  from 
  the 
  base 
  of 
  the 
  

   glabella 
  to 
  the 
  lateral 
  margins 
  of 
  the 
  head. 
  Eyes 
  large, 
  promi- 
  

   nent, 
  sublunate, 
  visual 
  surface 
  finely 
  reticulated.* 
  Movable 
  

   cheeks 
  subtriangular 
  in 
  form, 
  sloping 
  rapidly 
  from 
  the 
  base 
  

   of 
  the 
  eye 
  to 
  the 
  margin, 
  upper 
  angle 
  truncated 
  to 
  form 
  the 
  

   base 
  of 
  the 
  eye. 
  Facial 
  suture, 
  cutting 
  the 
  occipital 
  segment 
  at 
  

   its 
  posterior 
  edge 
  on 
  a 
  line 
  with 
  the 
  outer 
  margin 
  of 
  the 
  eye 
  

   thence 
  passing 
  obliquely 
  inward 
  to 
  the 
  posterior 
  angle 
  of 
  the 
  

   eye, 
  and 
  curving 
  outward 
  around 
  the 
  top 
  of 
  the 
  eye 
  to 
  the 
  

   anterior 
  angle, 
  it 
  curves 
  down, 
  with 
  a 
  slight 
  outward 
  obliquity, 
  

   to 
  the 
  inner 
  edge 
  of 
  the 
  border, 
  then 
  curving 
  forward 
  terminates 
  

   opposite 
  the 
  widest 
  part 
  of 
  the 
  glabella 
  upon 
  the 
  frontal 
  

   margin. 
  

   Thorax: 
  axial 
  lobe, 
  strongly 
  convex, 
  the 
  segments, 
  nine 
  

  

  *The 
  facets 
  are 
  arranged 
  upon 
  the 
  visual 
  surface 
  in 
  oblique 
  lines, 
  the 
  crossing 
  of 
  which 
  

   gives 
  the 
  reticulated 
  appearance. 
  The 
  outer 
  corneas 
  in 
  all 
  ^specimens 
  examined 
  were 
  

   absent, 
  which 
  left 
  the 
  facetted 
  surface 
  exposed. 
  The 
  facetted 
  surface 
  of 
  the 
  eye 
  of 
  

   Asaphus 
  platycephalus, 
  has 
  essentially 
  the 
  same 
  arrangement; 
  a 
  short 
  hexagonal 
  prism 
  

   extends 
  from 
  each 
  facet 
  to 
  the 
  inner 
  surface 
  of 
  the 
  eye, 
  perpendicular 
  to 
  the 
  sur- 
  

   face 
  of 
  the 
  facet 
  to 
  which 
  it 
  belongs. 
  The 
  prisms 
  are 
  closely 
  arranged, 
  each 
  side 
  resting 
  

   against 
  aside 
  of 
  each 
  of 
  the 
  six 
  surrounding 
  facets. 
  The 
  cornea 
  is 
  smooth 
  upon 
  its 
  outer 
  

   surface, 
  and 
  closely 
  and 
  firmly 
  attached 
  to 
  the 
  facetted 
  surface. 
  In 
  trilobites 
  that 
  have 
  

   attained 
  their 
  growth 
  the 
  cornea 
  is 
  usually 
  opaque 
  ; 
  in 
  younger 
  specimens 
  it 
  is 
  thinner 
  

   and 
  shows 
  the 
  facetted 
  surface 
  through 
  it, 
  or 
  else 
  bears 
  the 
  impression 
  of 
  the 
  facets 
  

   upon 
  its 
  outer 
  surface. 
  The 
  fortunate 
  discovery 
  of 
  a 
  specimen 
  showing 
  the 
  interior 
  of 
  

   both 
  eyes, 
  and, 
  on 
  a 
  section, 
  the 
  prismatic 
  structure 
  and 
  cornea, 
  has 
  enabled 
  me 
  to 
  

   determine 
  the 
  structure 
  of 
  the 
  eye 
  with 
  accuracy. 
  

  

  The 
  visual 
  surface 
  of 
  the 
  eye 
  of 
  Bathywus 
  longispinus 
  has 
  an 
  area 
  of 
  .0461 
  of 
  a 
  square 
  

   inch 
  ; 
  area 
  of 
  one 
  facet, 
  1-102400 
  of 
  a 
  square 
  inch 
  ; 
  number 
  of 
  facets 
  In 
  eye, 
  4,730. 
  Size 
  of 
  

   head; 
  length, 
  1 
  inch, 
  breadth, 
  XX 
  inches. 
  

  

  Area 
  of 
  visual 
  surface 
  of 
  the 
  eye 
  of 
  Asaphus 
  platycephalus, 
  .0736 
  of 
  a 
  square 
  inch 
  ; 
  area 
  

   of 
  one 
  facet, 
  1-103400 
  of 
  a 
  square 
  inch; 
  number 
  of 
  facets 
  in 
  eye, 
  7,536. 
  Size 
  of 
  head; 
  

   length, 
  IX 
  inches, 
  breadth, 
  2% 
  inches. 
  

  

  The 
  eye 
  of 
  Ceraurus 
  pleurexanthemus 
  was 
  found 
  to 
  have 
  four 
  hundred 
  facets 
  ; 
  the 
  

   arrangement 
  of 
  the 
  prisms 
  is 
  the 
  same 
  as 
  in 
  Asaphus 
  platycephalus. 
  Each 
  facet 
  is 
  

   slightly 
  convex 
  upon 
  its 
  outer 
  surface 
  ; 
  the 
  cornea 
  covering 
  the 
  facetted 
  surface 
  partakes 
  

   of 
  the 
  convexity 
  of 
  each 
  facet, 
  hence 
  the 
  peculiar 
  granulated 
  visual 
  surface, 
  noted 
  by 
  

   Professor 
  Hall, 
  Pal. 
  N. 
  Y, 
  Vol. 
  1, 
  p. 
  343 
  

  

  