﻿Report 
  of 
  the 
  State 
  Geologist. 
  1107 
  

  

  The 
  purely 
  conventional 
  value 
  of 
  family 
  designations 
  could 
  not 
  

   be 
  more 
  forcibly 
  illustrated 
  than 
  among 
  the 
  groups 
  now 
  under 
  

   consideration, 
  and 
  as 
  shown 
  by 
  the 
  following 
  facts 
  : 
  Among 
  the 
  

   Obolid^ 
  there 
  is, 
  in 
  a 
  certain 
  direction, 
  a 
  tendency 
  manifested 
  

   toward 
  the 
  formation 
  of 
  a 
  thickened 
  muscular 
  platform, 
  a 
  struc- 
  

   ture 
  to 
  which 
  we 
  have 
  already 
  referred 
  as 
  being 
  most 
  highly 
  

   developed 
  in 
  the 
  trimerellids 
  (Trimerella, 
  Dinobolus, 
  etc.). 
  This 
  

   is 
  seen 
  in 
  the 
  Cambrian 
  genus 
  Elkaiqa, 
  where 
  these 
  platforms 
  

   are 
  solid 
  muscular 
  thickenings 
  of 
  the 
  shell, 
  and 
  their 
  development 
  

   is 
  more 
  highly 
  advanced 
  in 
  Lakhmina, 
  also 
  of 
  Cambrian 
  age, 
  in 
  

   which 
  the 
  platforms 
  are 
  vaulted 
  or 
  excavated 
  beneath. 
  Dihtobo- 
  

   ltjs, 
  which 
  makes 
  its 
  appearance 
  in 
  the 
  early 
  faunas 
  of 
  the 
  

   Silurian 
  (Black 
  River 
  limestone), 
  is 
  a 
  large 
  shell 
  of 
  oboloid 
  

   exterior 
  and 
  having 
  these 
  platforms 
  well 
  developed, 
  sometimes 
  

   solid, 
  but 
  often 
  excavated. 
  This 
  genus 
  continues 
  its 
  existence 
  or 
  

   rather, 
  in 
  accordance 
  with 
  our 
  knowledge, 
  reappears 
  after 
  a 
  long 
  

   interval, 
  in 
  the 
  later 
  faunas 
  of 
  the 
  Upper 
  Silurian 
  in 
  a 
  fuller 
  

   manifestation 
  and 
  in 
  association 
  with 
  other 
  platform-bearing 
  

   genera, 
  Trimerella, 
  Monomerella, 
  Dinobolus. 
  

  

  Among 
  the 
  Lingulil\e 
  a 
  similar 
  tendency 
  to 
  the 
  formation 
  of 
  

   these 
  platforms 
  manifests 
  itself, 
  though 
  at 
  a 
  later 
  period 
  than 
  

   in 
  the 
  oboloids. 
  The 
  elementary 
  condition 
  of 
  development 
  

   is 
  expressed 
  in 
  Lingulops, 
  and 
  in 
  a 
  more 
  advanced 
  stage 
  in 
  

   Lingulasma, 
  of 
  the 
  middle 
  Silurian. 
  In 
  Trimerella, 
  Mono- 
  

   merella 
  and 
  Rhinoboltjs 
  of 
  the 
  later 
  Silurian 
  the 
  lingu- 
  

   loid 
  form 
  of 
  the 
  shell 
  is 
  retained 
  and 
  the 
  highest 
  stage 
  

   of 
  platform 
  development 
  attained. 
  These 
  shells 
  have 
  been 
  

   closely 
  studied 
  ; 
  our 
  knowledge 
  of 
  them 
  is 
  fairly 
  complete, 
  suffi- 
  

   cient 
  to 
  justify 
  us 
  in 
  the 
  conclusion 
  that 
  in 
  the 
  Trimerellidm 
  

   (of 
  Davidson 
  and 
  King; 
  a 
  very 
  natural 
  group, 
  including 
  

   Trimerella, 
  Monomerella, 
  Rhlnobolus 
  and 
  Dinobolus, 
  to 
  which 
  

   we 
  should 
  add 
  Lingulops 
  and 
  Lingulasma), 
  the 
  derivation 
  of 
  the 
  

   platform 
  has 
  been 
  along 
  two 
  distinct 
  and 
  convergent 
  phyletic 
  

   lines, 
  one 
  departing 
  from 
  the 
  oboloids, 
  the 
  other 
  from 
  the 
  

   linguloids. 
  The 
  lire 
  of 
  the 
  platform 
  bearing 
  In 
  articulates 
  ends 
  

   abruptly 
  and 
  finally 
  with 
  the 
  close 
  of 
  the 
  Silurian. 
  The 
  tend- 
  

   ency 
  to 
  produce 
  these 
  platforms, 
  which 
  was 
  then 
  common 
  to 
  the 
  

   oboloids 
  and 
  linguloids, 
  and 
  hence 
  a 
  heritage 
  from 
  common 
  

   ancestry 
  fails 
  to 
  manifest 
  itself 
  in 
  other 
  lines 
  of 
  departure 
  

  

  165 
  

  

  