﻿132 
  THE 
  EXTINCT 
  BATRACHIA, 
  REPTILIA 
  

  

  Euryslernum 
  differs 
  from 
  Chelydra 
  as 
  Osteopygis 
  does, 
  in 
  a 
  greater 
  coossification 
  oi 
  

   the 
  discal 
  and 
  marginal 
  bones 
  anteriorly 
  and 
  posteriorly. 
  It 
  differs 
  from 
  the 
  latter 
  in 
  the 
  

   stout 
  ambulatory 
  foot 
  like 
  that 
  of 
  the 
  Chelydra, 
  and 
  in 
  the 
  apparent 
  absence 
  of 
  the 
  

   temporal 
  osseous 
  roof, 
  which 
  I 
  suspect 
  Osteopygis 
  to 
  possess. 
  The 
  vertebral 
  dermal 
  

   scuta 
  are 
  wider 
  than 
  in 
  any 
  species 
  of 
  our 
  genera, 
  but 
  this 
  is 
  but 
  a 
  specific 
  character. 
  

  

  In 
  Idiochelys 
  the 
  marginal 
  bones 
  are 
  more 
  distinct 
  from 
  the 
  disc, 
  entirely 
  so 
  poste- 
  

   riorly; 
  there 
  are 
  numerous 
  omissions 
  of 
  the 
  vertebral 
  bones 
  of 
  the 
  carapace; 
  this 
  does 
  

   not 
  occur 
  among 
  our 
  species 
  so 
  far 
  as 
  known. 
  

  

  Hydropelta 
  is 
  in 
  general 
  much 
  like 
  Osteopygis, 
  but 
  in 
  it 
  the 
  hyo 
  and 
  hyposternal 
  

   bones, 
  have 
  an 
  articulation 
  by 
  gomphosis 
  with 
  the 
  third 
  and 
  tenth 
  marginals 
  respectively, 
  

   a 
  character 
  certainly 
  wanting 
  to 
  the 
  American 
  genera. 
  There 
  is 
  also 
  no 
  median 
  sternal 
  

   fontanelle, 
  which 
  is 
  present 
  in 
  our 
  types. 
  

  

  Platychelys 
  has 
  a 
  still 
  greater 
  union 
  of 
  carapacial 
  disc 
  and 
  margin; 
  only 
  three 
  ribs 
  

   have 
  free 
  extremities 
  on 
  each 
  side. 
  

  

  Our 
  forms 
  then 
  appear 
  to 
  differ 
  from 
  those 
  of 
  the 
  Jurassic 
  of 
  Europe, 
  up 
  to 
  the 
  

   present 
  time. 
  It 
  is 
  important 
  to 
  observe, 
  that 
  instead 
  of 
  being 
  of 
  marine 
  habit 
  as 
  has 
  

   been 
  supposed, 
  they 
  are 
  representatives 
  of 
  modern 
  fresh 
  water 
  species, 
  and 
  were 
  probably 
  

   inhabitants 
  of 
  brackish 
  estuaries 
  of 
  our 
  coast. 
  

  

  OSTEOPYGIS, 
  Cope. 
  

   Proceed. 
  Acad. 
  Nat. 
  Sciences, 
  Phila., 
  18G8, 
  147. 
  

  

  This 
  genus 
  has 
  been 
  characterized 
  above, 
  in 
  part. 
  It 
  has 
  the 
  usual 
  twenty-three 
  

   marginal 
  bones 
  with 
  ten 
  costal 
  bones 
  on 
  each 
  side, 
  nine 
  of 
  which 
  send 
  their 
  free 
  extremi- 
  

   ties 
  for 
  gomphosis 
  with 
  the 
  marginals. 
  Though 
  in 
  sutural 
  union 
  with 
  the 
  second 
  mar- 
  

   ginals, 
  each 
  sends 
  a 
  costal 
  process 
  into 
  a 
  corresponding 
  pit 
  of 
  the 
  latter. 
  The 
  inter- 
  

   costal 
  union 
  is 
  prolonged, 
  and 
  the 
  vertebral 
  scuta 
  are 
  rather 
  narrow, 
  and 
  do 
  not 
  therefore 
  

   extend 
  far 
  on 
  the 
  former. 
  The 
  four 
  posterior 
  marginals 
  are 
  prolonged 
  considerably 
  within 
  

   the 
  groove 
  marking 
  the 
  suture 
  of 
  the 
  costal 
  and 
  marginal 
  scuta. 
  The 
  posterior 
  marginal 
  

   plates 
  of 
  all 
  the 
  species 
  are 
  very 
  flat 
  and 
  expanded, 
  the 
  lateral 
  on 
  the 
  contrary 
  trigonal 
  

   or 
  subtrigonal 
  in 
  section, 
  with 
  a 
  distinct 
  inferior 
  plane. 
  

  

  The 
  accompanying 
  cut 
  gives 
  the 
  characters 
  of 
  the 
  genus 
  as 
  exhibited 
  in 
  the 
  carapace 
  

   of 
  the 
  typical 
  species. 
  The 
  dermal 
  sutures 
  are 
  omitted 
  from 
  the 
  left 
  side, 
  so 
  as 
  to 
  show 
  

   more 
  clearly 
  those 
  of 
  the 
  skeleton. 
  

  

  The 
  plastron 
  of 
  Osteopygis 
  is 
  more 
  like 
  that 
  of 
  Chelydra 
  than 
  any 
  other 
  known 
  

   genus. 
  The 
  hyo-,hypo, 
  and 
  xiphisternals 
  are 
  united 
  on 
  the 
  median 
  line 
  by 
  a 
  coarse 
  open 
  

   suture 
  as 
  in 
  that 
  genus, 
  and 
  arc 
  not 
  separated 
  as 
  in 
  Chclonc. 
  They 
  arc 
  much 
  more 
  united 
  

   than 
  in 
  Trionyx. 
  

  

  