﻿NEWTON 
  : 
  ON 
  nAETOMYA, 
  81 
  

  

  position 
  in 
  both 
  valves; 
  it 
  has, 
  moreover, 
  cardinal 
  teeth, 
  and 
  a 
  

   siphonal 
  inflection 
  which 
  is 
  deep, 
  angiilate, 
  and 
  sloping 
  downwards 
  

   from 
  the 
  posterior 
  adductor 
  scar, 
  being 
  nearly 
  parallel 
  with 
  the 
  

   ventral 
  margin. 
  The 
  adductor 
  scar 
  impressions 
  are 
  of 
  the 
  same 
  

   design 
  as 
  tliose 
  of 
  the 
  African 
  shell, 
  while 
  the 
  umbones 
  occupy 
  

   a 
  more 
  approximate 
  position 
  and 
  are 
  on 
  the 
  same 
  plane. 
  

  

  The 
  new 
  shell 
  has 
  similar 
  dorso-marginal 
  sulci 
  as 
  that 
  of 
  Raeta, 
  

   which 
  it 
  is 
  considered 
  would 
  be 
  for 
  the 
  reception 
  of 
  ligament 
  and 
  

   therefore 
  amphtdettc, 
  as 
  expressed 
  in 
  Neumayr's^ 
  terminology, 
  as- 
  

   opposed 
  to 
  opistJiodetic, 
  when 
  the 
  ligament 
  is 
  entirely 
  behind 
  the 
  

   umbones. 
  

  

  Again, 
  there 
  are 
  no 
  paired 
  laminae 
  to 
  receive 
  the 
  lateral 
  teeth 
  as 
  

   in 
  the 
  genus 
  Mactra 
  ; 
  these 
  are 
  also 
  absent 
  in 
  Raeta. 
  These 
  and 
  the 
  

   other 
  distinctions 
  already 
  referred 
  to 
  I 
  consider 
  of 
  sufficient 
  

   importance 
  for 
  regarding 
  Raetomya 
  under 
  the 
  new 
  family 
  designation 
  

   of 
  llaetomyidae. 
  In 
  like 
  manner 
  I 
  would 
  suggest 
  that 
  Raeta, 
  from 
  

   its 
  differences 
  to 
  Mactra, 
  in 
  contour, 
  its 
  unequal 
  adductor 
  scars, 
  and 
  

   the 
  possession 
  of 
  a 
  large 
  angulate 
  pallial 
  sinus, 
  should 
  be 
  placed 
  

   under 
  the 
  family 
  name 
  of 
  llaetida3 
  instead 
  of 
  Mactridae. 
  

  

  Dr. 
  Dall,'^ 
  among 
  other 
  investigators, 
  has 
  largely 
  increased 
  our 
  

   knowledge 
  on 
  the 
  morphology 
  of 
  the 
  Pelecypod 
  hinge, 
  his 
  earlier 
  

   studies 
  of 
  the 
  subject 
  having 
  been 
  published 
  in 
  our 
  Society's 
  

   Proceedings 
  as 
  a 
  preliminary 
  thesis 
  to 
  more 
  extended 
  observations 
  

   contained 
  in 
  his 
  great 
  memoir 
  on 
  The 
  Tertianj 
  Fauna 
  of 
  Florida. 
  

  

  In 
  these 
  works 
  the 
  true 
  "ligament" 
  is 
  regarded 
  as 
  external, 
  

   while 
  the 
  name 
  "resilium" 
  is 
  given 
  to 
  the 
  "relatively 
  or 
  actually 
  

   internal 
  bond 
  between 
  the 
  dorsal 
  margins 
  of 
  tlie 
  valves 
  " 
  which 
  by 
  

   older 
  authors 
  had 
  been 
  termed 
  "internal 
  cartilage" 
  or 
  ligament, 
  its 
  

   function 
  being 
  that 
  of 
  separating 
  the 
  valves 
  by 
  "expansional 
  

   elasticity". 
  Dr. 
  Dall 
  notes 
  the 
  well-recognized 
  difference 
  in 
  the 
  

   composition 
  of 
  these 
  substances, 
  the 
  ligament 
  proper 
  being 
  of 
  a 
  

   horny 
  natui'e 
  and 
  semi-translucent, 
  whereas 
  the 
  resilium 
  is 
  lamellar 
  

   or 
  composed 
  of 
  parallel 
  fibres, 
  which 
  give 
  a 
  pearly 
  sheen 
  to 
  its 
  

   broken 
  surface. 
  Such 
  structures 
  are, 
  however, 
  rarely 
  seen 
  in 
  the 
  

   fossil 
  state, 
  the 
  palaeontologist 
  having 
  only 
  calcareous 
  material 
  to 
  

   guide 
  him, 
  so 
  that 
  the 
  hard 
  chondrophore 
  plates 
  of 
  the 
  present 
  shell 
  

   between 
  which 
  the 
  resilium 
  formerly 
  resided 
  add 
  greatly 
  to 
  its 
  

   features 
  of 
  interest. 
  

  

  The 
  genus 
  Lovellia 
  was 
  founded 
  by 
  Mayer-Eymar^ 
  on 
  Lutraria 
  

   canalictdala 
  of 
  Say 
  * 
  belonging 
  to 
  the 
  recent 
  fauna 
  of 
  the 
  North 
  

   Atlantic, 
  which 
  itself 
  was 
  the 
  type 
  of 
  Gray's 
  genus 
  Raeta 
  ^ 
  ; 
  but 
  

   objecting 
  to 
  what 
  he 
  termed 
  " 
  noms 
  barbares" 
  Mayer-Eymar 
  set 
  

  

  1 
  Denkschr. 
  k. 
  Akad. 
  Wiss. 
  Wien, 
  vol. 
  Iviii, 
  1891, 
  p. 
  711. 
  

  

  " 
  Proc. 
  Malac. 
  See. 
  London, 
  vol. 
  i, 
  1893, 
  pp. 
  203-13, 
  and 
  Trans. 
  Wagner 
  Free 
  

  

  Inst. 
  Sci. 
  Philadelphia, 
  vol. 
  iii, 
  pts. 
  iii-v, 
  1895-1900. 
  

   ' 
  Vierteljahrsch. 
  Nat. 
  Ges. 
  Ziirich, 
  vol. 
  xii, 
  1867, 
  pp. 
  275, 
  286. 
  

   * 
  Journ. 
  Acad. 
  Nat. 
  Sci. 
  Philadelphia, 
  vol. 
  ii, 
  pt. 
  ii, 
  1822, 
  p. 
  311. 
  

   '" 
  Ann. 
  Mag. 
  Nat. 
  Hist., 
  ser. 
  II, 
  vol. 
  xi, 
  1853, 
  p. 
  43. 
  The 
  origin 
  of 
  the 
  name 
  

  

  Raeta 
  is 
  unknown 
  ; 
  it 
  was 
  probably 
  founded 
  in 
  honour 
  of 
  Sir 
  John 
  Eae, 
  

  

  the 
  Arctic 
  traveller. 
  

  

  