﻿VERTEBRATES. 
  229 
  

  

  obtains 
  in 
  forms 
  of 
  teeth 
  of 
  opposite 
  jaws 
  of 
  Tertiary 
  and 
  existing 
  

   Myliodonts. 
  

  

  The 
  affinities 
  of 
  the 
  Copodi 
  are 
  certainly 
  with 
  the 
  Myliodonts, 
  the 
  

   single 
  row 
  of 
  teeth, 
  spanning 
  the 
  jaws 
  like 
  paving 
  flags, 
  finding 
  a 
  

   somewhat 
  intimate 
  repetition 
  in 
  the 
  still 
  existing 
  genus 
  Aetobatis. 
  

   There 
  are, 
  however, 
  other 
  characters 
  that 
  offer 
  marked 
  contrasts 
  in 
  

   contradistinction 
  of 
  the 
  ancient 
  and 
  the 
  latter 
  modern 
  representa- 
  

   tive, 
  and 
  which 
  probably 
  should 
  be 
  interpreted 
  as 
  possessing 
  an 
  

   importance 
  even 
  of 
  greater 
  consequence 
  than 
  generic. 
  The 
  charac- 
  

   teristics 
  possessed 
  in 
  common 
  with 
  the 
  genus 
  Psammodus, 
  Agass., 
  

   show 
  unmistakable 
  family 
  relationship 
  with 
  the 
  latter, 
  while 
  structur- 
  

   ally 
  and 
  in 
  the 
  character 
  of 
  the 
  basal 
  portion 
  of 
  the 
  teeth 
  there 
  is 
  

   even 
  striking 
  contrast 
  to 
  the 
  features 
  noted 
  in 
  typical 
  Myliodonts. 
  

   We 
  are, 
  therefore, 
  of 
  the 
  opinion 
  that 
  the 
  family 
  distinctions 
  of 
  the 
  

   Palaeozoic 
  teeth 
  here 
  alluded 
  to 
  are 
  paramount 
  and 
  should 
  be 
  so 
  

   recognized 
  ; 
  hence 
  the 
  family 
  designation 
  Psammoclontid<e, 
  which 
  was 
  

   bestowed 
  by 
  Dr. 
  de 
  Koninck 
  on 
  this 
  group 
  of 
  ancient 
  selachian 
  re- 
  

   mains, 
  is 
  here 
  adopted. 
  

  

  The 
  genus 
  appears 
  to 
  be 
  restricted 
  to 
  the 
  earlier 
  groups 
  of 
  Car- 
  

   boniferous 
  formations 
  both 
  in 
  Europe 
  and 
  America. 
  

  

  Copodus 
  Van 
  Hornii, 
  St. 
  J. 
  and 
  W. 
  

  

  PI. 
  XX. 
  Pig. 
  2, 
  3. 
  

  

  Teeth 
  representing 
  the 
  forms 
  pertaining 
  respectively 
  to 
  the 
  upper 
  

   and 
  the 
  lower 
  jaws, 
  and 
  which 
  attain 
  medium 
  size. 
  

  

  Maxillary 
  teeth 
  Ungulate 
  in 
  outline, 
  very 
  gently 
  arched 
  from 
  

   within 
  outward. 
  Lateral 
  borders 
  nearly 
  straight, 
  gradually 
  con- 
  

   verging 
  and 
  angularly 
  rounded 
  into 
  the 
  anterior 
  margin, 
  posteriorly 
  

   terminating 
  in 
  a 
  produced 
  spur, 
  the 
  basal 
  portion 
  projecting 
  in 
  a 
  

   widish 
  rim 
  beyond 
  the 
  coronal 
  limits, 
  inferiorly 
  inbeveled 
  and 
  

   rounded 
  to 
  the 
  inferior 
  surface 
  ; 
  anterior 
  margin 
  regularly 
  and 
  mod- 
  

   erately 
  arched, 
  the 
  coronal 
  band 
  forming 
  a 
  coping 
  constituting 
  one- 
  

   fourth 
  or 
  one-third 
  of 
  the 
  depth, 
  and 
  well 
  defined 
  from 
  the 
  exca- 
  

   vated 
  basal 
  portion 
  ; 
  the 
  posterior 
  margin 
  approximately 
  conforms 
  

   to 
  the 
  curvature 
  of 
  the 
  opposite 
  margin, 
  and 
  is 
  similarly 
  marked, 
  

   the 
  channeled 
  basal 
  portion 
  in 
  both 
  instances 
  irregularly 
  vertically 
  

   pitted. 
  The 
  coronal 
  region 
  is 
  moderately 
  arched 
  transversely, 
  flat- 
  

   tened 
  along 
  the 
  median 
  line, 
  with 
  a 
  lunate 
  depressed 
  triturating 
  

   area 
  reaching 
  back 
  from 
  the 
  anterior 
  margin 
  a 
  greater 
  or 
  less 
  dis- 
  

  

  