﻿NEWTON 
  : 
  ON 
  RAETOMYA. 
  83 
  

  

  These 
  characters, 
  mostly 
  mentioned 
  in 
  the 
  original 
  account, 
  are 
  

   mainly 
  of 
  external 
  interest, 
  very 
  little 
  apparently 
  being 
  known 
  as 
  to 
  

   those 
  affecting 
  the 
  interior 
  of 
  the 
  valves. 
  The 
  new 
  and 
  better 
  

   preserved 
  material 
  from 
  Southern 
  jS^igeria 
  fortunately 
  supplies 
  some 
  

   important 
  details 
  of 
  internal 
  structure 
  which 
  contribute 
  to 
  the 
  value 
  

   of 
  the 
  species, 
  as 
  well 
  as 
  suggesting 
  its 
  recognition 
  as 
  the 
  type 
  of 
  

   a 
  new 
  genus 
  ; 
  the 
  emended 
  specific 
  characters 
  may 
  be 
  set 
  out 
  as 
  

   follows 
  : 
  — 
  

  

  Umbonal 
  region 
  well 
  elevated 
  and 
  arched; 
  summits 
  resting 
  on 
  the 
  

   sliell 
  margin, 
  opisthogyrous, 
  laterally 
  approximate 
  but 
  divided 
  in 
  

   age, 
  that 
  of 
  the 
  right 
  valve 
  always 
  slightly 
  raised 
  above 
  the 
  other. 
  

   Left 
  valve 
  with 
  a 
  strong 
  hinge-plate, 
  having 
  all 
  its 
  elements 
  

   coalescent 
  and 
  radiating 
  from 
  beneath 
  the 
  umbonal 
  region, 
  which 
  

   embrace: 
  — 
  a 
  horizontally 
  projecting 
  chondrophore 
  of 
  elongate 
  

   triangular 
  shape 
  and 
  slightly 
  concave 
  surface, 
  bordered 
  by 
  a 
  posterior 
  

   groove, 
  and 
  followed 
  by 
  a 
  steep, 
  oblique, 
  posterior 
  side 
  which 
  forms 
  

   part 
  of 
  a 
  wide 
  and 
  deep 
  dorso-marginal 
  sulcus, 
  which 
  probably 
  

   received 
  the 
  ligament 
  as 
  well 
  as 
  the 
  postero-lamellar 
  tooth 
  of 
  the 
  

   opposing 
  valve. 
  The 
  chondrophore 
  surface 
  is 
  covered 
  with 
  minute 
  

   and 
  closely-set 
  concentric 
  striations 
  crossed 
  by 
  radial 
  riblets, 
  and 
  

   bordering 
  three-fourths 
  of 
  its 
  anterior 
  side 
  (which 
  is 
  longest) 
  is 
  a 
  

   narrowly 
  elongate, 
  rounded, 
  funnel-shaped 
  cavity 
  marked 
  with 
  fine 
  

   concentric 
  and 
  radial 
  striations, 
  which 
  is 
  regarded 
  as 
  the 
  receptacle 
  

   for 
  the 
  anterior 
  part 
  of 
  the 
  resilium. 
  This 
  cavity 
  bears 
  an 
  antero- 
  

   marginal 
  ridge 
  which 
  may 
  probably 
  represent 
  the 
  so-called 
  anterior 
  

   lamellar 
  tooth 
  of 
  the 
  original 
  description 
  of 
  the 
  species 
  ; 
  then 
  

   follows 
  the 
  dorso-anterior 
  marginal 
  sulcus 
  which 
  holds 
  the 
  ligament. 
  

   The 
  basal 
  or 
  underneath 
  surface 
  of 
  the 
  hinge-plate 
  is 
  quite 
  smooth, 
  

   more 
  or 
  less 
  bulbous, 
  and 
  furrowed. 
  Right 
  valve 
  with 
  a 
  slightly 
  

   excavated, 
  triangularly-oval 
  chondrophore, 
  vertically 
  situated, 
  and 
  

   merged 
  in 
  the 
  sub-umbonal 
  region, 
  its 
  surface 
  similarl)^ 
  ornamented 
  

   to 
  that 
  of 
  the 
  other 
  chondrophore, 
  although 
  smoother 
  and 
  less 
  obvious. 
  

   The 
  posterior 
  border 
  of 
  this 
  cavity 
  is 
  obliquely 
  and 
  roundlj^ 
  ridged 
  

   to 
  fit 
  the 
  postero-lateral 
  groove 
  of 
  the 
  left 
  chondrophore, 
  this 
  being 
  

   followed 
  by 
  an 
  oblique 
  sulcus 
  and 
  a 
  prominently 
  oblique, 
  postero- 
  

   laminar 
  tooth, 
  to 
  which 
  succeeds 
  a 
  wide 
  and 
  flattened 
  internal 
  

   dorsal 
  margin. 
  Neither 
  of 
  the 
  valves 
  exhibits 
  any 
  indication 
  of 
  

   cardinal 
  teeth. 
  

  

  The 
  pallial 
  impression 
  as 
  seen 
  on 
  internal 
  casts 
  shows 
  a 
  large 
  and 
  

   nearly 
  circular 
  sinus, 
  an 
  adult 
  specimen 
  measuring 
  85 
  mm. 
  in 
  length 
  

   giving 
  the 
  dimensions 
  of 
  this 
  sinus 
  as 
  25 
  by 
  28 
  mm. 
  in 
  vertical 
  and 
  

   longitudinal 
  diameters 
  respectively. 
  

  

  The 
  adductor 
  scar 
  impressions 
  are 
  of 
  very 
  unequal 
  position 
  and 
  

   shape, 
  the 
  posterior 
  being 
  of 
  moderate 
  size, 
  oblong-pyriform, 
  

   concentrically 
  striated, 
  and 
  well 
  above 
  the 
  middle 
  of 
  the 
  shell 
  margin, 
  

   while 
  the 
  anterior 
  is 
  narrowly 
  elongate, 
  extending 
  to 
  nearly 
  the 
  

   antero- 
  ventral 
  curvature 
  of 
  the 
  valves, 
  as 
  in 
  Raeta. 
  

  

  The 
  sculpture 
  consists 
  of 
  rounded 
  plications 
  and 
  furrows 
  marked 
  

   with 
  extremely 
  fine 
  concentric 
  interlineations, 
  together 
  with 
  obscure 
  

   microscopical 
  radial 
  striations 
  which 
  enter 
  into 
  the 
  texture 
  of 
  the 
  

  

  