﻿AND 
  AVES 
  OF 
  NORTH 
  AMERICA. 
  39 
  

  

  penetrating 
  the 
  centrum, 
  which, 
  though 
  not 
  wanting 
  in 
  such 
  mammals 
  as 
  Basilosaurus 
  and 
  

   its 
  allies, 
  are 
  neither 
  so 
  numerous 
  nor 
  situate 
  so 
  near 
  the 
  neural 
  arch 
  as 
  here. 
  (See 
  Leidy's 
  

   fig. 
  11.) 
  

  

  The 
  ribs, 
  as 
  remarked 
  by 
  Dr. 
  Leidy, 
  are 
  remarkably 
  dense. 
  He 
  observes 
  that 
  " 
  from 
  

   the 
  solidity 
  of 
  structure 
  and 
  cylindroid 
  form 
  of 
  the 
  ribs, 
  I 
  suspect 
  Ischyrotherium 
  to 
  be 
  

   more 
  nearly 
  allied 
  to 
  the 
  Manatee 
  than 
  to 
  any 
  other 
  animal." 
  This 
  consideration 
  does 
  

   not 
  affect 
  the 
  affinities 
  here 
  accepted 
  as 
  true. 
  The 
  structure 
  is 
  remarkable, 
  and 
  differs 
  

   from 
  that 
  of 
  Manatus 
  and 
  Squalodon 
  much 
  as 
  Reptiles 
  do 
  from 
  Mammals, 
  in 
  its 
  homoge- 
  

   neity, 
  or 
  when 
  interruption 
  of 
  the 
  same 
  occurs, 
  in 
  its 
  appearance 
  as 
  irregularly 
  disposed 
  

   cells, 
  and 
  in 
  the 
  lack 
  of 
  a 
  concentric 
  structure 
  of 
  any 
  kind. 
  In 
  the 
  Mammalian 
  genera 
  in 
  

   question, 
  as 
  well 
  as 
  in 
  Basilosaurus* 
  this 
  concentric 
  structure 
  is 
  eccentric 
  in 
  relation 
  to 
  

   the 
  circumference 
  of 
  the 
  rib. 
  

  

  The 
  genus 
  Mesosaurus 
  Gervais, 
  according 
  to 
  plate 
  XLII 
  of 
  his 
  Zoologie 
  et 
  Palteontol- 
  

   ogie 
  Generale 
  (the 
  letter 
  press 
  has 
  not 
  yet 
  reached 
  me), 
  presents 
  ribs 
  of 
  similar 
  form 
  to 
  

   those 
  of 
  Ischyrosaurus, 
  but 
  whether 
  of 
  similar 
  structure 
  I 
  cannot 
  ascertain. 
  

  

  Leidy 
  concludes 
  his 
  description 
  of 
  this 
  genus 
  with 
  the 
  following 
  remarks 
  : 
  " 
  Although 
  

   I 
  have 
  supposed 
  the 
  remains 
  * 
  * 
  to 
  indicate 
  * 
  * 
  an 
  animal 
  allied 
  

   to 
  the 
  Manatee, 
  * 
  * 
  I 
  have 
  suspected 
  that 
  they 
  have 
  belonged 
  to 
  an 
  aquatic 
  

   reptile 
  unlike 
  any 
  known." 
  * 
  * 
  Entertaining 
  the 
  opinions 
  that 
  I 
  do 
  respecting 
  

   the 
  relations 
  of 
  the 
  genus, 
  I 
  have 
  thought 
  that 
  the 
  name 
  applied 
  by 
  Leidy, 
  which 
  is 
  appro- 
  

   priate 
  only 
  to 
  a 
  mammal, 
  should 
  be 
  changed. 
  I 
  therefore 
  call 
  it 
  Ischyrosaurus, 
  maintain- 
  

   ing 
  the 
  first 
  etymology 
  so 
  far 
  as 
  practicable. 
  

  

  I 
  refer 
  it 
  to 
  the 
  Sauropterygia, 
  as 
  the 
  parts 
  resemble 
  Plesiosaurus 
  more 
  nearly 
  than 
  

   those 
  of 
  any 
  other 
  American 
  genus. 
  The 
  density 
  of 
  the 
  osseous 
  structure 
  and 
  the 
  cylin- 
  

   dric 
  form 
  of 
  the 
  ribs, 
  will 
  distinguish 
  it 
  from 
  Plesiosaurus 
  ; 
  from 
  Polycotylus 
  the 
  form 
  of 
  

   the 
  vertebrae 
  separates 
  it 
  at 
  once. 
  

  

  I 
  suppose 
  that 
  this 
  type 
  may 
  have 
  been 
  of 
  estuary 
  habits, 
  and 
  took 
  its 
  food 
  in 
  proxim- 
  

   ity 
  to 
  land. 
  The 
  density 
  of 
  the 
  bones 
  is 
  not 
  known 
  in, 
  nor 
  is 
  it 
  appropriate 
  to, 
  animals 
  

   of 
  the 
  open 
  ocean. 
  The 
  presence 
  of 
  Hadrosaurus 
  (Thespesius) 
  occidentalis 
  in 
  the 
  same 
  

   beds, 
  is 
  further 
  evidence 
  of 
  the 
  proximity 
  of 
  land. 
  

  

  ISCHYROSAURUS 
  ANTIQUUS, 
  Leidy. 
  

  

  Ischyrotherium 
  antiquum, 
  Leidy. 
  Proc. 
  Ac. 
  Nat. 
  Sci., 
  Phila., 
  VIII, 
  p. 
  89, 
  Trans. 
  Amer. 
  Philos. 
  Soc. 
  18G0, 
  150 
  

   Tab. 
  X, 
  figs. 
  8-17. 
  

  

  The 
  dorsal 
  vertebrae 
  of 
  this 
  species 
  present 
  plane 
  articular 
  extremities. 
  The 
  centrum 
  is 
  not 
  constricted 
  medially, 
  

   but 
  presents 
  a 
  shallow 
  concavity 
  round 
  its 
  median 
  portions. 
  The 
  sutural 
  articulation 
  of 
  the 
  neural 
  arch 
  is 
  shallow, 
  

   sub-ovate, 
  and 
  extends 
  throughout 
  the 
  length 
  of 
  the 
  centrum. 
  The 
  diapophyses 
  are 
  compressed 
  cylindric. 
  The 
  

  

  * 
  See 
  Owen 
  on 
  this 
  genus. 
  

  

  