﻿HTB0CRINT7S, 
  BAEB0CKI2STDS, 
  AKD 
  HYBOCYSTITES. 
  311 
  

  

  radial 
  (fig. 
  19). 
  In 
  the 
  other 
  two 
  individuals, 
  however, 
  it 
  comes 
  

   far 
  down 
  onto 
  the 
  radial 
  (figs. 
  9, 
  13, 
  24), 
  and 
  is 
  visible 
  on 
  the 
  

   dorsal 
  aspect 
  of 
  the 
  calyx 
  (figs. 
  11, 
  22). 
  But 
  its 
  course 
  is 
  different 
  

   in 
  the 
  two 
  cases. 
  In 
  Wetherby's 
  specimen 
  it 
  bends 
  considerably 
  

   backwards, 
  and 
  terminates 
  above 
  the 
  posterior 
  basal 
  (figs. 
  11, 
  13) 
  ; 
  

   whereas 
  in 
  Wachsmuth's 
  specimen 
  its 
  distal 
  portion 
  turns 
  slightly 
  

   forwards, 
  so 
  as 
  to 
  end 
  a 
  little 
  way 
  in 
  front 
  of 
  the 
  suture 
  between 
  

   the 
  left 
  posterior 
  and 
  the 
  posterior 
  basals 
  (figs. 
  22, 
  24). 
  

  

  These 
  three 
  individuals 
  not 
  only 
  exhibit 
  a 
  great 
  amount 
  of 
  insta- 
  

   bility 
  in 
  the 
  characters 
  of 
  the 
  ambulacra, 
  but 
  the 
  component 
  plates 
  

   of 
  the 
  calyx 
  present 
  occasional 
  deviations 
  from 
  their 
  ordinary 
  

   symmetrical 
  form, 
  as 
  is 
  evident 
  from 
  a 
  glance 
  at 
  the 
  figures. 
  The 
  

   most 
  striking 
  case 
  is 
  on 
  the 
  right 
  side 
  of 
  Wachsmuth's 
  larger 
  speci- 
  

   men 
  (fig. 
  18), 
  its 
  radial 
  departing 
  considerably 
  from 
  the 
  hexagonal 
  

   form 
  which 
  it 
  presents 
  in 
  the 
  other 
  two 
  specimens 
  (figs. 
  12, 
  23) 
  ; 
  

   while 
  the 
  basals 
  are 
  correspondingly 
  modified, 
  and 
  the 
  sutures 
  

   between 
  them 
  obscured. 
  In 
  like 
  manner 
  the 
  suture 
  between 
  the 
  

   anal 
  plate 
  and 
  the 
  small 
  radial 
  is 
  obscured 
  in 
  Wetherby's 
  specimen 
  

   (fig. 
  13) 
  ; 
  and 
  the 
  upper 
  boundaries 
  of 
  the 
  radials 
  are 
  in 
  nearly 
  all 
  

   cases 
  very 
  difficult 
  to 
  define. 
  Occasionally, 
  however, 
  (figs. 
  14, 
  15, 
  

   16, 
  20, 
  21) 
  there 
  seem 
  to 
  be 
  traces 
  of 
  oral 
  plates 
  between 
  the 
  

   ambulacra, 
  somewhat 
  as 
  in 
  Cyathocrinus. 
  

  

  In 
  Mr. 
  AVachsmuth's 
  larger 
  specimen 
  (fig. 
  16) 
  there 
  are 
  traces 
  of 
  

   small 
  pits 
  in 
  the 
  summit, 
  as 
  to 
  the 
  nature 
  of 
  which 
  it 
  is 
  difficult 
  to 
  

   decide. 
  If 
  they 
  really 
  mean 
  any 
  thing, 
  and 
  the 
  summit 
  be 
  an 
  oral 
  

   pyramid, 
  they 
  may 
  possibly 
  represent 
  the 
  water-pores 
  like 
  those 
  in 
  

   the 
  Pentacrinoid 
  larva 
  of 
  Comatala. 
  I 
  can, 
  however, 
  find 
  no 
  trace 
  

   of 
  them 
  in 
  either 
  of 
  the 
  other 
  specimens, 
  and 
  have 
  therefore 
  con- 
  

   siderable 
  doubt 
  as 
  to 
  their 
  possessing 
  any 
  real 
  morphological 
  value. 
  

   Failing 
  these, 
  I 
  can 
  find 
  no 
  trace 
  of 
  any 
  respiratory 
  organ 
  in 
  

   Hybocystites; 
  and 
  I 
  am 
  therefore 
  less 
  disposed 
  than 
  is 
  Prof. 
  

   "Wetherby 
  to 
  refer 
  it 
  to 
  the 
  Cystidea, 
  though 
  its 
  appressed 
  ambu- 
  

   lacra 
  give 
  it 
  a 
  certain 
  resemblance 
  to 
  some 
  forms 
  of 
  that 
  group. 
  

   But 
  their 
  relation 
  to 
  the 
  radials 
  is 
  far 
  more 
  like 
  that 
  of 
  the 
  ambu- 
  

   lacra 
  of 
  the 
  Blastoids. 
  These, 
  however, 
  are 
  usually 
  broader 
  than 
  

   in 
  Hybocystites, 
  and 
  have 
  various 
  accessory 
  structures 
  that 
  are 
  as 
  

   yet 
  unknown 
  in 
  this 
  type. 
  Of 
  the 
  ambulacra 
  of 
  the 
  Silurian 
  

   Stephanocrinus 
  we 
  unfortunately 
  know 
  but 
  little. 
  They 
  were 
  at 
  

   any 
  rate 
  narrow, 
  and 
  possibly 
  devoid 
  of 
  hydrospires. 
  Trcostocrinus 
  

   Reimvardtii, 
  the 
  earliest 
  Blastoid 
  which 
  is 
  satisfactorily 
  known, 
  

   has 
  narrow 
  linear 
  ambulacra 
  not 
  unlike 
  those 
  of 
  Hyhocystites 
  ; 
  but 
  

   they 
  cover 
  the 
  hydrospiral 
  tubes, 
  as 
  to 
  the 
  presence 
  of 
  which 
  in 
  

   Hybocystites 
  we 
  know 
  absolutely 
  nothing. 
  The 
  latter 
  type 
  differs 
  

   from 
  all 
  known 
  Blastoids 
  in 
  the 
  extension 
  of 
  the 
  ambulacra 
  onto 
  

   the 
  basal 
  plates 
  ; 
  while 
  its 
  calyx 
  presents 
  the 
  most 
  striking 
  analogy 
  

   with 
  that 
  of 
  Hybocrinus. 
  

  

  It 
  differs 
  from 
  the 
  Crinoids, 
  however, 
  in 
  the 
  absence 
  of 
  true 
  

   arms, 
  and 
  in 
  the 
  singular 
  relations 
  of 
  its 
  ambulacra. 
  The 
  course 
  of 
  

   three 
  of 
  them, 
  up 
  one 
  face 
  of 
  the 
  radial 
  extension 
  and 
  down 
  the 
  

   other, 
  may 
  perhaps 
  be 
  explained 
  bv 
  a 
  reference 
  to 
  Cupressocrinas. 
  

  

  y2 
  

  

  