﻿I. 
  DAVIDSON 
  AND 
  W. 
  KING 
  ON 
  THE 
  TKIMERELLIDJ-:. 
  

  

  129 
  

  

  In 
  certain 
  species 
  (Monomerella 
  prised) 
  the 
  hinge 
  of 
  the 
  brachial 
  

   valve 
  or, 
  rather, 
  we 
  believe, 
  its 
  outside, 
  is 
  strongly 
  striated 
  trans- 
  

   versely: 
  this 
  peculiarity 
  may 
  be 
  considered 
  the 
  same 
  as 
  the 
  trans- 
  

   verse 
  lines 
  on 
  the 
  areal 
  borders. 
  In 
  some 
  cases 
  the 
  point 
  of 
  the 
  

   umbo 
  of 
  the 
  brachial 
  valve 
  is 
  replaced 
  by 
  a 
  cavity 
  (Mon. 
  Walm- 
  

   stedti, 
  PI. 
  XVII. 
  fig. 
  3), 
  apparently 
  due 
  to 
  erosion 
  ; 
  in 
  others 
  (Tri- 
  

   merella 
  ohioensis, 
  PL 
  XVI. 
  figs. 
  5 
  & 
  6) 
  it 
  is 
  rounded. 
  

  

  As 
  regards 
  the 
  parts 
  belonging 
  to 
  their 
  interior, 
  the 
  Trimerellids 
  

   are 
  peculiarly 
  distinguished. 
  In 
  the 
  medio-longitudinal 
  region 
  of 
  the 
  

   posterior 
  half 
  of 
  both 
  valves 
  there 
  is 
  what 
  may 
  be 
  called 
  a 
  platform 
  

   (j 
  and 
  e 
  /)j 
  large 
  or 
  considerably 
  elevated 
  in 
  some 
  species 
  (PI. 
  XIV. 
  

   figs. 
  2, 
  3, 
  6, 
  &c), 
  small 
  and 
  even 
  excavated 
  in 
  others 
  (PI. 
  XVI. 
  fig. 
  8, 
  

   PI. 
  XVIII. 
  fig. 
  13, 
  PI. 
  XIX. 
  fig. 
  4). 
  In 
  most 
  cases 
  the 
  largest 
  and 
  

   most 
  elevated 
  platform 
  occurs 
  in 
  the 
  brachial 
  valve. 
  Generally 
  both 
  

   valves 
  have 
  the 
  platform 
  medio-longitudinally 
  depressed 
  on 
  the 
  top, 
  

   and 
  rounded 
  at 
  the 
  sides, 
  thereby 
  giving 
  it 
  a 
  biconvex 
  surface 
  : 
  it 
  also 
  

   presents 
  characters 
  which, 
  while 
  constant 
  in 
  certain 
  genera 
  and 
  spe- 
  

   cies, 
  undergo 
  marked 
  modifications 
  in 
  others. 
  Thus 
  in 
  most 
  Trime- 
  

   rellas 
  each 
  platform 
  is 
  distinguished 
  by 
  two 
  tubular 
  cavities 
  — 
  plat- 
  

   form 
  vaidts 
  (k, 
  k) 
  — 
  short, 
  or 
  nearly 
  obsolete 
  in 
  certain 
  species, 
  and 
  

   in 
  others 
  running 
  underneath 
  the 
  platform 
  in 
  its 
  entire 
  length 
  : 
  the 
  

   vaults 
  are 
  separated 
  by 
  a 
  median 
  partition 
  (PI. 
  XVI. 
  figs. 
  1, 
  2, 
  3). 
  In 
  

   a 
  few 
  species 
  the 
  platform 
  is 
  solid, 
  or 
  a 
  little 
  raised 
  above 
  the 
  inner 
  

   surface 
  of 
  the 
  valves, 
  with 
  the 
  slightest 
  indication 
  of 
  vaults 
  (PI. 
  XVI. 
  

   figs. 
  8, 
  9). 
  In 
  Monomerella 
  this 
  part, 
  though 
  well 
  developed, 
  is 
  not 
  

   vaulted 
  in 
  either 
  valve 
  ; 
  it 
  is 
  the 
  same 
  in 
  Dinololus. 
  

  

  Returning 
  to 
  Trimerella, 
  the 
  median 
  partition 
  is 
  prolonged 
  into 
  

   the 
  anterior 
  half 
  of 
  the 
  shell 
  under 
  the 
  form 
  of 
  a 
  median 
  plate 
  (1, 
  Z). 
  

   It 
  characterizes 
  both 
  valves, 
  but 
  is 
  always 
  the 
  largest 
  or 
  longest 
  in 
  

   the 
  brachial 
  valve 
  (Pi. 
  XIV. 
  figs. 
  2, 
  3, 
  &c). 
  Some 
  congeneric 
  spe- 
  

   cies 
  appear 
  to 
  have 
  had 
  no 
  plate 
  (PI. 
  XVI. 
  figs. 
  8, 
  9). 
  

  

  To 
  complicate 
  the 
  camerated 
  character 
  of 
  the 
  Trimerellids, 
  in 
  the 
  

   species 
  which 
  have 
  a 
  cardinal 
  buttress 
  this 
  part 
  divides 
  the 
  interior 
  

   of 
  the 
  umbo 
  (of 
  the 
  pedicle-valve) 
  into 
  two 
  hollow 
  sections 
  or 
  

   umbonal 
  chambers 
  (i), 
  large 
  and 
  wide-mouthed 
  in 
  the 
  Monomerellas, 
  

   long 
  and 
  tubular 
  in 
  Trimerella 
  Lindstrbmi. 
  There 
  are 
  species, 
  how- 
  

   ever, 
  belonging 
  to 
  both 
  these 
  genera, 
  also 
  Dinobolus 
  generally, 
  with- 
  

   out 
  umbonal 
  chambers 
  ; 
  though 
  in 
  some 
  cases 
  indications 
  of 
  them 
  

   occur. 
  Occasionally 
  there 
  are 
  appearances 
  of 
  them 
  in 
  the 
  brachial 
  

   valve 
  : 
  an 
  unusually 
  developed 
  case 
  occurs 
  in 
  this 
  valve 
  of 
  Trimerella 
  

   Lindstromi, 
  as 
  shown 
  by 
  the 
  pair 
  of 
  postlateral 
  prominences 
  (casts) 
  

  

  limited 
  a 
  view, 
  probably 
  arising 
  from 
  an 
  examination 
  of 
  a 
  specimen 
  with 
  long 
  

   lamelliform 
  borders, 
  such 
  as 
  occur, 
  one 
  on 
  each 
  side, 
  on 
  the 
  hinge 
  of 
  T. 
  ohio- 
  

   ensis 
  (PL 
  XVI. 
  fig. 
  7). 
  In 
  this 
  species, 
  however, 
  there 
  is 
  a 
  deepish 
  hollow 
  on 
  the 
  

   inner 
  or 
  posterior 
  side 
  of 
  each 
  border, 
  and 
  seemingly 
  corresponding 
  callosities 
  

   on 
  the 
  hinge 
  (where 
  angulated) 
  of 
  the 
  pedicle-valve 
  : 
  the 
  latter 
  may 
  have 
  be- 
  

   longed 
  to 
  the 
  deltidial 
  ridges. 
  So 
  variable 
  are 
  the 
  cardinal 
  sockets 
  and 
  cardinal 
  

   callosities, 
  that 
  we 
  are 
  quite 
  prepared 
  to 
  meet 
  with 
  specimens 
  having 
  lamelli- 
  

   form 
  borders 
  developed 
  or 
  elongated 
  somewhat 
  as 
  represented 
  by 
  Lindstrom. 
  

  

  

  