﻿822 
  Forty-seventh 
  Report 
  on 
  the 
  State 
  Museum. 
  

  

  biconvex 
  valves, 
  well-defined 
  cardinal 
  areas 
  and 
  elongate 
  hinge- 
  

   line 
  ; 
  producing, 
  in 
  effect, 
  a 
  generally 
  orthoid 
  expression, 
  both 
  of 
  

   interior 
  and 
  exterior. 
  This 
  is 
  the 
  condition 
  of 
  Billingsella 
  of 
  

   the 
  Cambrian, 
  Orthis 
  loricula 
  and 
  0. 
  deflecta 
  of 
  the 
  Trenton 
  

   group, 
  and 
  0. 
  f 
  laurentina 
  of 
  the 
  Hudson 
  . 
  iver 
  fauna, 
  and 
  it 
  is 
  

   continued 
  without 
  essential 
  modification, 
  except 
  in 
  the 
  gradual 
  

   contraction 
  of 
  the 
  pedicle-cavity 
  and 
  deltidium, 
  into 
  Strophomena 
  

   of 
  the 
  Silurian, 
  its 
  allies 
  and 
  successors, 
  Orthothetes 
  of 
  the 
  

   Devonian 
  and 
  Derbya 
  of 
  the 
  Carboniferous, 
  Hipparionyx 
  

   Triplecia, 
  Streptorhynohus, 
  etc., 
  into 
  Lept^na, 
  Rafinisqulna, 
  

   Stropheodonta, 
  Plectambonites, 
  Chonetes 
  and 
  Productus. 
  

  

  The 
  tendency 
  to 
  contract 
  the 
  pedicle-cavity 
  and 
  deltidium 
  pre- 
  

   sents 
  its 
  extreme 
  manifestation 
  in 
  the 
  Devonian 
  forms 
  of 
  Stroph- 
  

   eodonta, 
  Strophonella 
  and 
  Leptostrophia, 
  where 
  it 
  has 
  become 
  

   almost, 
  and 
  sometimes 
  quite 
  obliterated, 
  and 
  the 
  entire 
  pedicle 
  

   and 
  umbonal 
  cavity 
  filled 
  with 
  testaceous 
  secretions. 
  Such 
  filling 
  

   can 
  occur 
  only 
  in 
  a 
  discarded 
  and 
  useless 
  space, 
  after 
  the 
  pedicle 
  

   has 
  ceased 
  to 
  be 
  functional 
  L 
  morphological 
  consideration 
  of 
  

   much 
  importance 
  presents 
  itself 
  here, 
  as 
  well 
  as 
  in 
  any 
  other 
  

   groups 
  of 
  genera 
  where 
  the 
  shells 
  attain 
  great 
  size. 
  The 
  evi- 
  

   dence 
  is 
  very 
  direct 
  from 
  the 
  study 
  of 
  the 
  structural 
  features 
  as 
  

   given 
  above, 
  that 
  the 
  entire 
  muscular 
  system 
  on 
  the 
  ventral 
  side 
  

   of 
  the 
  body 
  is, 
  in 
  primitive 
  forms, 
  inserted 
  upon 
  the 
  base 
  of 
  

   the 
  pedicle-cavity. 
  This 
  is 
  apparent 
  from 
  a 
  study 
  of 
  such 
  a 
  shell 
  

   as 
  Orthis 
  callactis, 
  where 
  it 
  is 
  perfectly 
  clear 
  that 
  no 
  muscular 
  

   bands 
  were 
  attached 
  to 
  the 
  pedicle- 
  valve 
  ouside 
  the 
  limits 
  of 
  this 
  

   strong 
  and 
  condensed 
  posterior 
  area, 
  which 
  is 
  but 
  a 
  sessile 
  spondy- 
  

   lium. 
  The 
  contraction 
  of 
  this 
  pedicle-cavity 
  is 
  accompanied 
  by 
  

   (whether 
  in 
  relation 
  of 
  cause 
  to 
  effect 
  can 
  not 
  be 
  stated) 
  a 
  diffusion 
  

   of 
  the 
  area 
  of 
  muscular 
  attachment, 
  and 
  when 
  the 
  shells 
  are 
  large, 
  

   as 
  in 
  Strophomena, 
  Rafinesqtjina, 
  Stropheodonta, 
  Orthothetes, 
  

   Derbya, 
  etc., 
  the 
  necessity 
  for 
  powerful 
  muscles 
  or 
  some 
  similar 
  

   cause, 
  magnifies 
  this 
  expansion 
  of 
  the 
  muscular 
  area 
  until 
  the 
  

   original 
  contents 
  of 
  the 
  pedicle 
  cavity 
  may 
  be 
  represented 
  by 
  

   enormous 
  muscles 
  whose 
  scars 
  extend 
  almost 
  to 
  the 
  anterior 
  

   margin 
  of 
  the 
  valve, 
  as 
  in 
  Hipparionyx 
  and 
  Rhipidomella. 
  

  

  In 
  this 
  great 
  group 
  of 
  genera 
  there 
  are 
  two 
  types 
  of 
  contour, 
  

   one 
  as 
  in 
  Lept^ena 
  being 
  normally 
  con 
  vexo 
  concave, 
  that 
  is, 
  with 
  

   the 
  pedicle- 
  valve 
  convex 
  and 
  the 
  brachial 
  valve 
  parallel 
  to 
  it 
  and 
  

  

  