﻿164 
  MB. 
  F. 
  A. 
  BATHEE 
  OX 
  TKIGOXOCRIXtTS 
  EEOiT 
  

  

  variety, 
  this 
  method 
  frequently 
  results 
  in 
  a 
  series 
  of 
  forms 
  recog- 
  

   nized 
  as 
  species 
  and 
  even 
  as 
  genera. 
  

  

  (iii) 
  "fusion"' 
  differs 
  from 
  the 
  former 
  methods 
  in 
  that 
  no 
  part 
  is 
  

   lost 
  ; 
  the 
  fundamental 
  morphology 
  of 
  the 
  calyx 
  remains 
  unchanged, 
  

   and 
  internal 
  organs, 
  such 
  as 
  the 
  canals, 
  bear 
  witness 
  to 
  a 
  former 
  

   different 
  state 
  of 
  things. 
  Thus, 
  in 
  many 
  Palaeozoic 
  crinoids 
  the 
  

   basals, 
  typically 
  five 
  in 
  number, 
  become 
  by 
  concrescence 
  4, 
  3, 
  2. 
  or 
  

   even 
  1, 
  while 
  their 
  relations 
  to 
  the 
  rest 
  of 
  the 
  calyx 
  may 
  be 
  un- 
  

   altered*. 
  Compare 
  also 
  the 
  fused 
  basals 
  of 
  Bathycrinus 
  and 
  Rhizo- 
  

   crinusf. 
  Of 
  gradual 
  fission 
  I 
  know 
  no 
  instances, 
  but 
  it 
  may 
  take 
  

   place 
  for 
  all 
  that. 
  In 
  Marsupites 
  testudinarius, 
  v. 
  Schloth., 
  the 
  

   dorso-central 
  is 
  occasionally 
  found 
  divided 
  into 
  three 
  or 
  four 
  irre- 
  

   gular 
  plates, 
  and 
  in 
  Apiocrinus 
  fission 
  of 
  the 
  top 
  stem-joint 
  is 
  not 
  

   rare 
  ; 
  but 
  these 
  variations 
  are 
  confined 
  to 
  individuals. 
  Fission 
  in 
  

   a 
  horizontal 
  plane 
  is, 
  of 
  course, 
  common, 
  but 
  I 
  am 
  here 
  referring 
  to 
  

   changes 
  that 
  take 
  place 
  parallel 
  to 
  the 
  long 
  axis 
  of 
  the 
  crinoid. 
  

  

  The 
  interest 
  of 
  Trigonocrinus 
  in 
  the 
  present 
  connexion 
  lies 
  in 
  the 
  

   fact 
  that 
  it 
  is 
  an 
  example 
  of 
  all 
  three 
  methods 
  of 
  change. 
  As 
  such, 
  

   it 
  stands 
  alone 
  ; 
  and 
  only 
  by 
  appreciating 
  their 
  distinctness 
  can 
  we 
  

   understand 
  how 
  it 
  has 
  reached 
  its 
  present 
  structure. 
  Dr. 
  P. 
  H. 
  

   Carpenter 
  suggested 
  to 
  me 
  a 
  comparison 
  with 
  Allagecrinus; 
  the 
  

   changes 
  in 
  that 
  form 
  are 
  of 
  remarkable 
  interest, 
  none 
  of 
  which 
  was 
  

   lost 
  by 
  the 
  authors 
  of 
  the 
  genus 
  i. 
  A 
  detailed 
  comparison 
  shows 
  

   many 
  curious 
  points 
  of 
  resemblance 
  between 
  the 
  two 
  forms 
  — 
  e. 
  g. 
  

   fused 
  basals, 
  separate 
  radials, 
  interradial 
  sutures 
  in 
  grooves, 
  atrophy 
  

   of 
  a 
  radial 
  with 
  imperfect 
  articular 
  surface, 
  absence 
  of 
  central 
  canal 
  

   in 
  radials 
  of 
  young 
  Allagecrinus 
  (see 
  PI. 
  VI. 
  figs. 
  17, 
  IS). 
  But, 
  

   as 
  the 
  authors 
  point 
  out, 
  the 
  characters 
  of 
  Allagecrinus 
  are 
  those 
  of 
  

   an 
  undeveloped 
  rather 
  than 
  of 
  a 
  degenerate 
  form 
  : 
  further 
  the 
  devia- 
  

   tion 
  from 
  pentamerous 
  symmetry 
  is 
  brought 
  about 
  by 
  Atrophy 
  

   alone. 
  

  

  The 
  evolution 
  of 
  Trigonocrinus. 
  on 
  the 
  other 
  hand, 
  appears 
  to 
  have 
  

   been 
  as 
  follows 
  : 
  — 
  Starting 
  from 
  a 
  normal 
  Crinoid 
  (fig. 
  2, 
  I, 
  p. 
  165) 
  

   with 
  five 
  basals, 
  1, 
  2, 
  3, 
  4, 
  5, 
  and 
  five 
  first 
  radials, 
  a, 
  b, 
  c, 
  d, 
  e, 
  a 
  

   Sport 
  effected 
  the 
  loss 
  of 
  one 
  radial, 
  b, 
  and 
  its 
  corresponding 
  basal, 
  2. 
  

   Thus 
  arose 
  the 
  form 
  in 
  fig. 
  2, 
  II, 
  with 
  four 
  radials 
  and 
  four 
  basals. 
  

   One 
  side 
  of 
  the 
  calyx 
  then 
  increased 
  at 
  the 
  expense 
  of 
  the 
  other 
  side 
  ; 
  

   basals 
  3 
  and 
  5 
  slightly 
  diminished 
  in 
  size, 
  while 
  basal 
  4 
  atrophied 
  

   to 
  a 
  considerable 
  extent 
  ; 
  radials 
  a 
  and 
  c 
  remained 
  large, 
  while 
  

   radials 
  e 
  and 
  d 
  atrophied 
  : 
  such 
  a 
  stage 
  is 
  represented 
  in 
  III 
  ; 
  no 
  

   actual 
  form 
  of 
  this 
  composition 
  is 
  known, 
  but 
  it 
  is 
  closely 
  parallelled 
  

  

  * 
  See 
  Ch. 
  Wachsmuth 
  and 
  F. 
  Springer, 
  ' 
  Eerision 
  of 
  the 
  Palasocrinoidea,' 
  

   author's 
  copy, 
  part 
  i. 
  pp. 
  16-19, 
  and 
  Proc. 
  Acad. 
  Mat. 
  Sci. 
  Philad. 
  vol. 
  for 
  1879, 
  

   part 
  iii. 
  pp. 
  240-242. 
  Philadelphia, 
  1880. 
  

  

  + 
  P. 
  H. 
  Carpenter, 
  ' 
  Challenger 
  ! 
  Eeport. 
  Zoology, 
  vol. 
  xi. 
  part 
  xxxii. 
  . 
  . 
  . 
  

   The 
  Stalked 
  Crinoids, 
  see 
  pp. 
  226-228 
  and 
  248-253.' 
  London, 
  1884. 
  

  

  J 
  P. 
  H. 
  Carpenter 
  and 
  E. 
  Etheridge, 
  Jun., 
  li 
  Contributions 
  to 
  the 
  Study 
  of 
  

   the 
  British 
  Palaeozoic 
  Crinoids. 
  X/o. 
  1. 
  On 
  Allagecrinus, 
  the 
  representative 
  of 
  

   a 
  new 
  Family 
  from 
  the 
  Carboniferous-Limestone 
  series 
  of 
  Scotland.'' 
  Ann. 
  & 
  

   Mag. 
  Nat. 
  Hist. 
  ser. 
  5, 
  vol. 
  ^ii. 
  pp. 
  281-293, 
  pis. 
  xt. 
  xvi. 
  London, 
  1881. 
  

  

  