﻿* 
  PRELIMINARY 
  NOTICE 
  OF 
  THE 
  DISCOVERY 
  

  

  OF 
  THE 
  REMAINS 
  OF 
  THE 
  NATATORY 
  AND 
  BRANCHIAL 
  

   APPENDAOES 
  OF 
  TRILOBITES. 
  

  

  By 
  C. 
  D. 
  WALCOTT. 
  

  

  In 
  making 
  sections 
  of 
  trilobites, 
  from 
  the 
  Trenton 
  Lime- 
  

   stone, 
  at 
  Trenton 
  Falls, 
  N. 
  Y., 
  specimens 
  were 
  used, 
  which 
  

   were 
  embedded 
  in 
  rocks 
  varying 
  in 
  their 
  character, 
  from 
  the 
  

   common 
  gray 
  to 
  the 
  dark, 
  fine 
  grained, 
  bluish-gray 
  limestone. 
  

   In 
  only 
  two 
  layers 
  of 
  a 
  fine, 
  bluish-gray 
  sediment, 
  were 
  

   trilobites 
  found, 
  which 
  had 
  structural 
  remains 
  preserved 
  

   beneath 
  the 
  dorsal 
  shell. 
  

  

  The 
  remains, 
  with 
  the 
  exception 
  of 
  the 
  hypostoma, 
  appear 
  

   to 
  be 
  of 
  a 
  semi-calcified 
  nature, 
  as 
  if 
  a 
  thin 
  membrane 
  had 
  

   inclosed 
  them 
  until 
  the 
  organic 
  substance 
  had 
  been 
  replaced. 
  

   Frequently 
  all 
  traces 
  of 
  structure 
  are 
  destroyed 
  by 
  the 
  pres- 
  

   ence 
  of 
  crystalized 
  calcite. 
  The 
  species 
  Asaphus 
  megistos, 
  

   Calymene 
  senaria, 
  Ceraurus 
  pleureccanthemus, 
  and 
  Acidaspis 
  

   Trentonensis, 
  have 
  afforded 
  evidence 
  of 
  the 
  presence 
  of 
  

   organic 
  structure 
  beneath 
  the 
  dorsal 
  shell 
  ; 
  Ceraurus 
  pleurex- 
  

   antliemus 
  and 
  Calymene 
  senaria 
  furnishing 
  the 
  greater 
  por- 
  

   tion 
  of 
  the 
  evidence. 
  

  

  Twenty-one 
  individuals 
  of 
  the 
  species 
  Ceraurus 
  pleurex- 
  

   anthemus, 
  show 
  the 
  axial 
  appendages. 
  Each 
  segment 
  of 
  the 
  

   thorax, 
  of 
  the 
  pygidium 
  and 
  the 
  three 
  posterior 
  of 
  the 
  head, 
  

   have 
  a 
  pair 
  beneath 
  the 
  axial 
  processes. 
  The 
  appendages 
  

   beneath 
  the 
  thorax 
  and 
  pygidium, 
  appear 
  to 
  be, 
  from 
  trans- 
  

   verse 
  and 
  longitudinal 
  sections, 
  short, 
  unjointed, 
  cylindrical 
  

   supports 
  for 
  swimming 
  lobes, 
  or 
  the 
  remains 
  of 
  rudimentary, 
  

   ambulatory 
  legs. 
  The 
  three 
  pair 
  of 
  appendages 
  beneath 
  the 
  

   head, 
  present 
  an 
  obscurely 
  jointed 
  structure. 
  Twelve 
  speci- 
  

   mens 
  of 
  Calymene 
  senaria 
  show 
  the 
  axial 
  appendages. 
  One 
  

   individual 
  has 
  twenty-one 
  on 
  one 
  side. 
  The 
  appendages 
  in 
  

  

  