﻿156 
  T. 
  DAVIDSON 
  AND 
  W. 
  KING 
  ON 
  THE 
  TRIMERELLID^E. 
  

  

  of 
  it, 
  apparently 
  sockets, 
  in 
  the 
  brachial 
  valve 
  of 
  M. 
  Walmstedti. 
  

   The 
  cardinal 
  process, 
  slightly 
  developed, 
  is 
  only 
  known 
  to 
  us 
  in 
  this 
  

   species. 
  We 
  entertain 
  considerable 
  doubt 
  as 
  to 
  the 
  transverse 
  lines 
  

   occurring 
  apparently 
  on 
  the 
  hinge 
  of 
  M. 
  prisca 
  being 
  the 
  repre- 
  

   sentatives 
  of 
  this 
  part 
  : 
  their 
  position 
  behind 
  the 
  crown 
  of 
  the 
  crescent 
  

   strongly 
  favours 
  our 
  scepticism. 
  The 
  genus 
  includes 
  the 
  following 
  

   species. 
  

  

  Monomereela 
  prisca, 
  Billings. 
  PI. 
  XYII. 
  figs. 
  5-8. 
  

  

  Monomerella 
  prisca, 
  Bill. 
  Can. 
  Nat. 
  vol. 
  vi. 
  p. 
  221, 
  29 
  Dec. 
  1871. 
  

   Dav. 
  & 
  King, 
  Remarks 
  on 
  the 
  genera 
  Trimerella 
  &c, 
  Beport 
  

   of 
  Brighton 
  Meeting 
  of 
  Brit. 
  Assoc. 
  20th 
  August 
  1872 
  ; 
  Annals 
  

   & 
  Mag. 
  Nat. 
  Hist. 
  Oct. 
  1872, 
  and 
  Geol. 
  Mag. 
  Oct. 
  1872. 
  

   Pedicle-valve 
  obscurely 
  triangular 
  ; 
  with 
  rounded 
  angles 
  ; 
  longer 
  

   than 
  wide 
  ; 
  greatest 
  breadth 
  at 
  about 
  two 
  thirds 
  of 
  its 
  length 
  from 
  

   the 
  beak 
  ; 
  tapering 
  posteriorly 
  ; 
  very 
  much 
  flattened 
  ; 
  convex 
  only 
  

   at 
  the 
  sides 
  : 
  umbo 
  almost 
  straight 
  ; 
  about 
  one 
  third 
  of 
  the 
  length 
  

   of 
  the 
  valve 
  : 
  area 
  broadly 
  triangular 
  : 
  deltidial 
  ridges 
  moderately 
  

   wide 
  : 
  areal 
  borders 
  narrow 
  ; 
  raised 
  ; 
  strongly 
  marked 
  with 
  trans- 
  

   verse 
  lines 
  : 
  deltidial 
  slope 
  and 
  cardinal 
  facet 
  well 
  developed 
  : 
  hinge, 
  

   in 
  its 
  central 
  portion, 
  much 
  depressed: 
  umbonal 
  chambers 
  wide- 
  

   mouthed 
  : 
  cardinal 
  buttress 
  strong 
  ; 
  prolonged 
  to 
  some 
  distance 
  along 
  

   the 
  bottom 
  of 
  the 
  valve 
  : 
  platform 
  moderately 
  large 
  ; 
  obtusely 
  an- 
  

   gular 
  in 
  front 
  ; 
  with 
  one 
  of 
  its 
  sets 
  of 
  scars 
  striated 
  longitudinally, 
  

   and 
  strongly 
  sulcated 
  obliquely. 
  Brachial 
  valve 
  almost 
  circular; 
  

   nearly 
  straight 
  in 
  front 
  ; 
  evenly 
  convex 
  ; 
  and 
  deeper 
  than 
  the 
  oppo- 
  

   site 
  one: 
  hinge 
  slightly 
  raised 
  or 
  convex 
  in 
  its 
  middle 
  portion; 
  

   transversely 
  striated 
  on 
  what 
  appears 
  to 
  be 
  its 
  outer 
  or 
  exposed 
  por- 
  

   tion 
  : 
  crescent 
  in 
  parts 
  strongly 
  marked 
  : 
  platform 
  large 
  ; 
  V-shaped 
  ; 
  

   round 
  and 
  projecting 
  at 
  its 
  antemedian 
  point 
  ; 
  very 
  slightly 
  elevated 
  

   above 
  the 
  surface 
  of 
  the 
  valve 
  ; 
  its 
  scars 
  tolerably 
  well 
  shown. 
  

  

  This 
  species 
  is 
  strongly 
  marked 
  by 
  characteristic 
  features, 
  espe- 
  

   cially 
  its 
  wide 
  deltidium. 
  Its 
  various 
  muscular 
  scars 
  are 
  generally 
  

   well 
  displayed. 
  In 
  the 
  brachial 
  valve 
  three 
  pairs 
  (anterior, 
  median, 
  

   and 
  lateral) 
  are 
  well 
  seen 
  on 
  the 
  platform 
  : 
  the 
  anterior 
  pair 
  is 
  de- 
  

   pressed 
  below 
  the 
  median 
  pair. 
  The 
  subcardial 
  scar, 
  w, 
  is 
  rather 
  

   conspicuous 
  in 
  the 
  umbonal 
  cavity. 
  The 
  hinge 
  appears 
  to 
  be 
  un- 
  

   usually 
  slight 
  in 
  depth, 
  showing 
  nothing 
  more 
  than 
  the_ 
  crown 
  of 
  

   the 
  crescent, 
  which 
  forms 
  a 
  deepish 
  furrow. 
  On 
  the 
  outside 
  of 
  the 
  

   hinge 
  there 
  is 
  a 
  strongly 
  striated 
  surface 
  ; 
  the 
  striae 
  parallel 
  and 
  

   running 
  transversely 
  : 
  probably 
  this 
  feature 
  represents 
  the 
  epidermal 
  

   lines 
  on 
  the 
  area 
  of 
  the 
  corresponding 
  valve 
  of 
  Lingula*. 
  The 
  an- 
  

   terior 
  portion 
  of 
  the 
  crescent, 
  apparently.its 
  ends, 
  widens 
  out 
  a 
  little, 
  

   and 
  presents 
  a 
  compound 
  appearance. 
  A 
  pair 
  of 
  umbo-lateral 
  scars, 
  

   as, 
  lies 
  within 
  the 
  crescent, 
  a 
  member 
  on 
  each 
  side, 
  at 
  its 
  departure 
  

   from 
  the 
  hinge. 
  

  

  The 
  pedicle-valve 
  also 
  shows 
  its 
  scars 
  for 
  the 
  most 
  part 
  rather 
  

   obviously 
  : 
  two 
  pairs, 
  one 
  behind 
  the 
  other, 
  occur 
  on 
  the 
  platform 
  ; 
  

   * 
  Our 
  figure 
  does 
  not 
  represent 
  the 
  striae 
  so 
  fully 
  as 
  could 
  be 
  wished 
  ; 
  they 
  

   are 
  in 
  reality 
  more 
  numerous, 
  forming 
  a 
  deeper 
  band, 
  and 
  run 
  out 
  straighter. 
  

  

  