﻿Report 
  of 
  the 
  State 
  Geologist. 
  831 
  

  

  of 
  the 
  descending 
  lamellae 
  of 
  the 
  loop 
  beyond 
  the 
  point 
  of 
  their 
  

   recurvature 
  into 
  the 
  ascending 
  lamellae. 
  What 
  is 
  thus 
  true 
  of 
  

   Zygospira 
  we 
  must 
  assume 
  to 
  have 
  been 
  equally 
  true 
  of 
  all 
  the 
  

   spire 
  bearers, 
  and 
  the 
  analogies 
  thus 
  established 
  bet 
  wee 
  n 
  them 
  

   and 
  the 
  loop-bearing 
  shells 
  are 
  these: 
  The 
  entire 
  loop 
  in 
  Die- 
  

   lasmas, 
  Cryptonella, 
  etc., 
  corresponds 
  to 
  that 
  portion 
  of 
  the 
  

   brachidium 
  in 
  the 
  Helicopegmata 
  which 
  lies 
  behind 
  the 
  anterior 
  

   basal 
  edges 
  of 
  the 
  jugum; 
  the 
  descending 
  lamellae 
  of 
  the 
  former 
  

   represent 
  only 
  the 
  posterior 
  portion 
  of 
  the 
  primary 
  lamellae 
  of 
  

   the 
  latter, 
  while 
  the 
  ascending 
  lamellae 
  and 
  transverse 
  connecting 
  

   band 
  of 
  the 
  Ancylobrachia 
  are 
  the 
  equivalent 
  of 
  the 
  jugum 
  in 
  the 
  

   spire-bearers. 
  The 
  spirals, 
  however, 
  are 
  a 
  later 
  development 
  in 
  

   the 
  individual, 
  and 
  are 
  hence, 
  undoubtedly, 
  a 
  subsequent 
  phyletic 
  

   condition. 
  Hence 
  it 
  is 
  inferred 
  that 
  the 
  spire-bearing 
  derived 
  

   their 
  brachidia 
  from 
  a 
  primitive 
  terebratuloid 
  condition, 
  and 
  this 
  

   derivation 
  has 
  been 
  effected 
  by 
  growth 
  with 
  accompanying 
  

   resorption. 
  

  

  The 
  progressive 
  modification 
  of 
  the 
  loop 
  in 
  the 
  recent 
  Tere- 
  

   bratellids 
  by 
  the 
  resorption 
  of 
  calcareous 
  tissue 
  in 
  the 
  growth 
  of 
  

   the 
  individual 
  is 
  a 
  well-known 
  fact 
  which 
  has 
  invited 
  the 
  study 
  

   of 
  many 
  investigators. 
  In 
  such 
  forms 
  this 
  modification 
  is 
  extreme, 
  

   and 
  is 
  unquestionably 
  complicated 
  by 
  the 
  intimate 
  connection 
  of 
  

   the 
  loop 
  with 
  the 
  median 
  septum 
  of 
  the 
  brachial 
  valve. 
  With 
  

   the 
  single 
  exception 
  of 
  Tropidoleptus 
  there 
  is 
  among 
  the 
  

   palaeozoic 
  genera, 
  no 
  clear 
  evidence 
  that 
  the 
  median 
  septum 
  has 
  

   shared 
  iu, 
  or 
  contributed 
  to 
  the 
  growth-modifications 
  of 
  the 
  

   brachial 
  supports 
  ; 
  nevertheless, 
  the 
  outcome 
  and 
  final 
  result 
  of 
  

   this 
  growth, 
  with 
  modification, 
  in 
  the 
  most 
  progressed 
  forms 
  

   of 
  Terebratella 
  and 
  such 
  palaeozoic 
  genera 
  as 
  Dielasma, 
  

   Cryptonella, 
  Harttista, 
  etc., 
  is 
  the 
  same. 
  

  

  Progressive 
  modification 
  of 
  the 
  brachial 
  supports 
  in 
  both 
  the 
  

   Helic 
  pegmata 
  and 
  palaeozoic 
  Ancylobrachia 
  being 
  now 
  fully 
  

   established, 
  it 
  is 
  interesting 
  to 
  observe 
  that 
  the 
  primitive 
  condi- 
  

   tion 
  of 
  the 
  loop, 
  as 
  in 
  Dielasma 
  turgida, 
  is 
  one 
  of 
  simple 
  apposi- 
  

   tion 
  of 
  the 
  two 
  short 
  brachial 
  processes, 
  at 
  their 
  expanded 
  anterior 
  

   extremities, 
  having 
  the 
  expression 
  of 
  the 
  mature 
  loop 
  in 
  the 
  

   genera 
  Centronella, 
  RENssELiERiA, 
  Selenella, 
  etc. 
  A 
  simple 
  

   step 
  further 
  back 
  would 
  afford 
  a 
  condition 
  in 
  which 
  the 
  brachial 
  

  

  