﻿158 
  T. 
  DAVIDSON 
  AND 
  W. 
  KING 
  ON 
  THE 
  TRIMERELLIDiE. 
  

  

  appear, 
  a 
  rounded 
  cardinal 
  prominence 
  ; 
  and 
  it 
  is 
  excavated 
  on 
  each 
  

   side, 
  or 
  where 
  the 
  dentary 
  sockets 
  occur 
  in 
  TrimereUa. 
  Instead 
  of 
  

   the 
  outside 
  of 
  the 
  hinge 
  being 
  transversely 
  striated, 
  it 
  shows 
  the 
  

   centre 
  of 
  the 
  umbo 
  excavated. 
  

  

  The 
  hinge-characters 
  of 
  the 
  pedicle-valve 
  resemble 
  those 
  of 
  M. 
  

   prisca. 
  The 
  cardinal 
  facet 
  is 
  similarly 
  situated, 
  equally 
  large, 
  and 
  

   shows 
  traces 
  of 
  the 
  lozenge, 
  as 
  well 
  as 
  the 
  crown 
  of 
  the 
  crescent. 
  

   The 
  platform 
  (except 
  in 
  being 
  more 
  elevated) 
  and 
  the 
  umbonal 
  

   chambers 
  are 
  also 
  in 
  close 
  agreement 
  with 
  those 
  in 
  the 
  latter 
  spe- 
  

   cies. 
  The 
  crescent 
  is 
  not 
  conspicuously 
  marked 
  in 
  either 
  valve. 
  

  

  Very 
  perfect 
  specimens, 
  in 
  separate 
  valves, 
  of 
  this 
  remarkable 
  

   species 
  were 
  discovered, 
  as 
  far 
  back 
  as 
  1867, 
  by 
  Dr. 
  Lindstrom, 
  at 
  

   Klinteberg, 
  Central 
  Gotland, 
  in 
  a 
  rock 
  considered 
  to 
  belong 
  to 
  the 
  

   Aymestry 
  formation. 
  These 
  specimens 
  are 
  preserved 
  in 
  the 
  Museum 
  

   of 
  Wisby 
  ; 
  and 
  other 
  examples 
  may 
  be 
  seen 
  in 
  that 
  of 
  Stockholm. 
  

   Some 
  internal 
  casts 
  of 
  the 
  same 
  species 
  were 
  found 
  by 
  Herr 
  Fegroeus 
  

   in 
  the 
  same 
  formation 
  in 
  the 
  island 
  of 
  Faro, 
  north 
  of 
  the 
  mainland 
  

   of 
  Gotland. 
  We 
  are 
  also 
  under 
  obligations 
  to 
  Prof. 
  F. 
  Schmidt, 
  of 
  

   St. 
  Petersburg, 
  for 
  kindly 
  enabling 
  us 
  to 
  add 
  copies 
  of 
  sketches 
  of 
  

   casts, 
  probably 
  representing 
  the 
  species 
  under 
  description, 
  which 
  

   were 
  found 
  by 
  Count 
  Keyserling 
  at 
  Kerkaw, 
  in 
  Livonia, 
  Russia, 
  in 
  

   rocks 
  considered 
  to 
  be 
  the 
  equivalent 
  of 
  the 
  Wenlock 
  Group 
  in 
  

   England 
  : 
  the 
  original 
  sketches 
  were 
  found 
  among 
  the 
  papers 
  left 
  

   by 
  the 
  late 
  Dr. 
  Pander. 
  

  

  Monomerella 
  Lindstrcemi, 
  Dav. 
  & 
  King. 
  PI. 
  XVII. 
  figs. 
  9, 
  9 
  a. 
  

  

  Pedicle-valve 
  transversely 
  elliptical 
  : 
  umbo 
  moderately 
  produced 
  : 
  

   umbonal 
  cavities 
  shallow, 
  and 
  wide-mouthed 
  : 
  cardinal 
  buttress 
  rather 
  

   . 
  stout 
  ; 
  attenuated 
  ; 
  considerably 
  elongated 
  in 
  advance 
  of 
  the 
  hinge 
  : 
  

   platform 
  long, 
  and 
  wide 
  ; 
  the 
  elongated 
  portion 
  of 
  the 
  buttress 
  passing 
  

   nearly 
  to 
  its 
  front 
  edge 
  ; 
  striated 
  longitudinally 
  ; 
  somewhat 
  rugose 
  

   transversely 
  ; 
  its 
  front 
  edge 
  obtusely 
  angulated. 
  Brachial 
  valve 
  trans- 
  

   versely 
  elliptical 
  in 
  outline 
  ; 
  flatly 
  rounded 
  in 
  the 
  middle 
  : 
  platform 
  

   with 
  anterior 
  lobe 
  a 
  little 
  more 
  elevated 
  than 
  the 
  lateral 
  lobes. 
  

  

  We 
  are 
  only 
  acquainted 
  with 
  this 
  shell 
  from 
  a 
  single 
  cast 
  showing 
  

   both 
  valves 
  in 
  conjunction, 
  and 
  measuring 
  11 
  lines 
  in 
  length 
  by 
  17 
  

   in 
  breadth 
  and 
  5 
  in 
  depth. 
  We 
  therefore 
  can 
  only 
  describe 
  it 
  im- 
  

   perfectly 
  : 
  but 
  the 
  character 
  and 
  shape 
  of 
  the 
  cast 
  leave 
  no 
  doubt 
  of 
  

   its 
  belonging 
  to 
  a 
  well-marked 
  species 
  — 
  and 
  different 
  from 
  either 
  of 
  

   the 
  preceding 
  in 
  being 
  much 
  wider 
  than 
  long, 
  and 
  more 
  compressed. 
  

  

  Monomerella 
  Lindstrcemi 
  was 
  procured 
  by 
  its 
  discoverer 
  (after 
  

   whom 
  it 
  has 
  been 
  named) 
  from 
  a 
  calcareous 
  bed 
  of 
  the 
  age 
  of 
  the 
  

   Wenlock 
  formation 
  in 
  the 
  neighbourhood 
  of 
  Wisby, 
  in 
  Gotland. 
  

  

  Monomerella 
  orbicularis, 
  Billings. 
  PL 
  XVII. 
  fig. 
  10. 
  

  

  Monomerella 
  orbicularis, 
  Bill. 
  The 
  Canadian 
  Naturalist, 
  vol. 
  vi. 
  

  

  p. 
  221, 
  29th 
  Dec. 
  1871 
  ; 
  Dav. 
  & 
  King, 
  Eeport 
  of 
  Brit. 
  Assoc. 
  

  

  Meeting, 
  Brighton, 
  20th 
  August 
  1872. 
  

  

  " 
  Broadly 
  ovate, 
  nearly 
  circular, 
  lenticular, 
  both 
  valves 
  moderately 
  

  

  convex. 
  The 
  casts 
  seem 
  to 
  show 
  that 
  a 
  thin 
  plate 
  extends 
  forwards 
  

  

  