﻿American 
  Ordovician 
  Lichadidcc. 
  41 
  

  

  back 
  on 
  the 
  front 
  of 
  the 
  thorax, 
  and 
  the 
  complete 
  speci- 
  

   men 
  might 
  show 
  eleven 
  segments. 
  The 
  normal 
  nnmber 
  

   of 
  thoracic 
  segments 
  in 
  Lichadidse 
  evidently 
  is 
  eleven. 
  

   Eleven 
  thoracic 
  segments 
  appear 
  in 
  that 
  specimen 
  of 
  

   Platylichas 
  (?) 
  halli 
  which 
  was 
  figured 
  by 
  Hall 
  errone- 
  

   ously 
  under 
  the 
  species 
  then 
  called 
  Acrolichas 
  trenton- 
  

   ensis 
  (Pal. 
  New 
  York, 
  1, 
  pi. 
  64, 
  fig. 
  le, 
  1847). 
  Eleven 
  

   thoracic 
  segments 
  occur 
  in 
  the 
  Silurian 
  species 
  Arctinu- 
  

   rus 
  boltoni 
  (Bigsby) 
  and 
  Dicranopeltis 
  nereus 
  (Hall), 
  

   both 
  figured 
  by 
  Hall; 
  and 
  also 
  in 
  Gorydocephalus 
  pal- 
  

   matus 
  (Barrande) 
  and 
  Dicranopeltis 
  scabra 
  (Beyrich), 
  

   both 
  figured 
  by 
  Barrande. 
  

  

  The 
  free 
  cheeks 
  of 
  Platylichas 
  (?) 
  halli 
  Foerste 
  have 
  

   been 
  found 
  attached 
  to 
  the 
  cranidium 
  (PL 
  II, 
  fig. 
  8A) 
  ; 
  

   this 
  falcate 
  type 
  of 
  free 
  cheek 
  appears 
  common 
  among 
  

   the 
  Lichadidse. 
  The 
  very 
  imperfectly 
  preserved 
  free 
  

   cheeks 
  belonging 
  to 
  the 
  type 
  of 
  Acrolichas 
  cornutus 
  are 
  

   interpreted 
  as 
  similar 
  in 
  form. 
  One 
  free 
  cheek 
  of 
  Acro- 
  

   lichas 
  cucullus 
  (PI. 
  I, 
  fig. 
  ID) 
  from 
  the 
  Kimmswick 
  lime- 
  

   stone 
  of 
  Illinois 
  shows 
  the 
  same 
  tendency 
  toward 
  a 
  

   falcate 
  outline. 
  

  

  The 
  hypostoma 
  of 
  a 
  number 
  of 
  species 
  of 
  Acrolichas 
  is 
  

   known, 
  but 
  has 
  not 
  yet 
  furnished 
  material 
  for 
  generic 
  

   differentiation 
  of 
  species. 
  That 
  of 
  Acrolichas 
  cucullus, 
  

   figured 
  on 
  Plate 
  I, 
  is 
  well 
  preserved. 
  (Bull. 
  Denison 
  

   Univ., 
  19, 
  1919, 
  pi. 
  17, 
  figs. 
  3A, 
  B; 
  pi. 
  18, 
  fig. 
  6.) 
  

  

  15. 
  Notes 
  on 
  American 
  species 
  of 
  Acrolichas. 
  

  

  On 
  the 
  preceding 
  pages 
  attention 
  has 
  been 
  concentrated 
  

   on 
  the 
  generic 
  differences 
  presented 
  by 
  American 
  Ordo- 
  

   vician 
  Lichads, 
  especially 
  on 
  the 
  proposed 
  genus 
  Acro- 
  

   lichas. 
  It 
  was 
  not 
  intended 
  to 
  present 
  here 
  all 
  of 
  the 
  

   information 
  so 
  far 
  accumulated 
  regarding 
  the 
  individual 
  

   species. 
  Only 
  a 
  few 
  of 
  the 
  observations 
  regarded 
  as 
  

   most 
  interesting 
  are 
  recorded 
  here. 
  

  

  Acrolichas 
  juhesii, 
  PI. 
  II, 
  figs. 
  1A, 
  B. 
  The 
  transverse 
  

   groove 
  noted 
  by 
  Billings 
  (Pal. 
  Foss., 
  1, 
  1865, 
  p. 
  335) 
  may 
  

   indicate 
  the 
  line 
  of 
  separation 
  between 
  the 
  first 
  and 
  sec- 
  

   ond 
  pairs 
  of 
  lateral 
  glabellar 
  lobes. 
  

  

  Acrolichas 
  minganensis, 
  PL 
  III, 
  figs. 
  1 
  A, 
  B, 
  C. 
  Ray- 
  

   mond 
  (Ann. 
  Carnegie 
  Museum, 
  7, 
  1910, 
  p. 
  72, 
  pi. 
  18, 
  fig. 
  

   6 
  ; 
  pi. 
  19, 
  fig. 
  14) 
  figured 
  two 
  small 
  cranidia 
  from 
  a 
  local- 
  

   ity 
  near 
  Chazy, 
  New 
  York, 
  in 
  which 
  the 
  glabellar 
  fur- 
  

  

  