﻿476 
  REPORT 
  OF 
  COMMISSIONER 
  OF 
  FISH 
  AND 
  FISHERIES. 
  [50] 
  

  

  iu 
  three 
  well-marked 
  stages, 
  starting 
  with 
  Halitherium 
  and 
  ending 
  with 
  

   Manatee. 
  The 
  extreme 
  degeneracy 
  of 
  the 
  pelvis 
  of 
  the 
  latter 
  it 
  would 
  

   seem 
  is 
  in 
  keeping 
  with 
  the 
  undeveloped 
  flukes 
  of 
  this 
  type, 
  which 
  are 
  

   mere 
  rounded 
  expansions 
  of 
  the 
  tail, 
  which 
  seems 
  to 
  be 
  simply 
  flattened 
  

   and 
  widened 
  posteriorly 
  into 
  a 
  sort 
  of 
  spatulate 
  form, 
  as 
  in 
  Fig. 
  20, 
  

   Plate 
  III, 
  showing 
  in 
  the 
  outer 
  outline 
  the 
  form 
  of 
  the 
  adult 
  and 
  in 
  the 
  

   inner 
  outline 
  that 
  of 
  the 
  tail 
  of 
  the 
  embryo, 
  both 
  being 
  quite 
  unlike 
  the 
  

   tail 
  of 
  Halicore, 
  Plate 
  III, 
  Fig. 
  22, 
  and 
  Rhytina 
  with 
  their 
  pointed 
  

   flukes. 
  

  

  That 
  Halitherium 
  ever 
  possessed 
  external 
  limbs 
  appears 
  to 
  me 
  to 
  be 
  

   exceedingly 
  doubtful 
  inasmuch 
  as 
  its 
  femur 
  is 
  more 
  rudimentary 
  than 
  

   in 
  Balcena 
  mysticetus, 
  and 
  no 
  tibial 
  rudiment 
  seems 
  to 
  be 
  developed. 
  

  

  There 
  is 
  no 
  dorsal 
  fin 
  developed 
  in 
  any 
  one 
  of 
  the 
  three 
  genera 
  of 
  

   sirenians 
  which 
  have 
  fallen 
  under 
  the 
  observation 
  of 
  naturalists. 
  

  

  As 
  to 
  the 
  affinities 
  of 
  the 
  sirenians, 
  I 
  think 
  it 
  very 
  doubtful 
  if 
  they 
  

   are 
  to 
  be 
  regarded 
  as 
  having 
  descended 
  from 
  the 
  same 
  mammalian 
  type 
  

   as 
  the 
  cetaceans, 
  for, 
  with 
  the 
  exception 
  of 
  the 
  degenerate 
  pelvis 
  and 
  

   distal 
  remnants 
  of 
  hind 
  limbs, 
  they 
  diverge 
  from 
  the 
  normal 
  type 
  far 
  

   less 
  than 
  do 
  the 
  cetaceans 
  ; 
  in 
  fact, 
  relatively 
  but 
  little 
  more 
  in 
  other 
  

   respects 
  than 
  do 
  the 
  Pinnipedia. 
  That 
  it 
  is 
  possible 
  that 
  they 
  were 
  

   differentiated 
  by 
  a 
  process 
  similar 
  to 
  that 
  which 
  has 
  brought 
  about 
  the 
  

   modification 
  of 
  the 
  cetaceans, 
  but 
  from 
  a 
  quite 
  distinct 
  form, 
  I 
  think 
  

   quite 
  conceivable. 
  Indeed 
  it 
  is 
  quite 
  easy 
  to 
  understand 
  that 
  a 
  perfectly 
  

   similar 
  change 
  might 
  be 
  induced 
  in 
  two 
  types 
  originally 
  very 
  greatly 
  

   dissimilar 
  through 
  the 
  long-continued 
  action 
  of 
  similar 
  influences 
  af- 
  

   fecting 
  the 
  functional 
  adaptation 
  of 
  the 
  hind 
  limbs, 
  as 
  already 
  suggested 
  

   in 
  the 
  case 
  of 
  Ichthyosaurus. 
  

  

  The 
  length 
  of 
  the 
  free 
  parts 
  of 
  the 
  pectoral 
  limbs 
  of 
  the 
  foetal 
  Dugong 
  

   described 
  by 
  Halting 
  was 
  almost 
  exactly 
  half 
  of 
  the 
  total 
  width 
  across 
  

   both 
  flukes, 
  the 
  length 
  of 
  the 
  former 
  being 
  5 
  centimeters, 
  and 
  the 
  trans- 
  

   verse 
  width 
  of 
  the 
  latter 
  10.3 
  centimeters. 
  This 
  is 
  a 
  very 
  suggestive 
  

   correspondence, 
  but 
  need 
  not 
  be 
  insisted 
  upon 
  as 
  indicating 
  anything 
  

   like 
  so 
  near 
  a 
  likeness 
  between 
  the 
  manus 
  and 
  pes 
  as 
  in 
  Cetacea, 
  be- 
  

   cause 
  the 
  fore 
  limb 
  in 
  Sirenians 
  has 
  the 
  arm 
  bones 
  better 
  developed 
  

   than 
  in 
  the 
  former 
  and 
  extended 
  outward 
  farther 
  beyond 
  the 
  level 
  of 
  

   the 
  common 
  integumentary 
  covering 
  of 
  the 
  animal. 
  The 
  nails 
  are 
  also 
  

   more 
  or 
  less 
  well 
  developed 
  on 
  the 
  manus 
  in 
  the 
  manatee. 
  

  

  The 
  smallest 
  foetal 
  sirenian 
  of 
  which 
  I 
  have 
  been 
  able 
  to 
  find 
  figures 
  

   and 
  a 
  description 
  is 
  by 
  Prof. 
  B. 
  G. 
  Wilder.* 
  This 
  specimen, 
  of 
  which 
  I 
  

   reproduce 
  Wilder's 
  original 
  figures, 
  measured 
  2.3 
  inches 
  from 
  the 
  vertex 
  

   to 
  root 
  of 
  tail, 
  3.7 
  inches 
  if 
  fully 
  extended. 
  Greatest 
  width 
  of 
  tail 
  11 
  

   millimeters 
  or 
  nearly 
  one-half 
  inch. 
  A 
  view 
  of 
  the 
  hinder 
  part 
  of 
  this 
  

   embryo, 
  Fig. 
  20, 
  Plate 
  III, 
  from 
  below 
  shows 
  that 
  the 
  trunk 
  is 
  much 
  

   more 
  abruptly 
  swollen 
  at 
  the 
  point 
  in 
  front 
  of 
  where 
  the 
  tail 
  begins 
  

  

  * 
  On 
  a 
  foetal 
  manatee 
  and 
  cetacean, 
  with 
  remarks 
  upon 
  the 
  affinities 
  and 
  ancestry 
  

   of 
  the 
  Sireuia, 
  Ainer. 
  Jonrn. 
  Sci. 
  and 
  Arts, 
  3d 
  ser. 
  X, 
  1S75, 
  pp. 
  105-114, 
  plate 
  VIII. 
  

  

  