﻿436 
  REPORT 
  OF 
  COMMISSIONER 
  OF 
  FISH 
  AND 
  FISHERIES. 
  [10] 
  

  

  ougbt 
  to, 
  as 
  longitudinal 
  ridges 
  or 
  folds 
  extending 
  the 
  whole 
  length 
  of 
  

   the 
  tail, 
  as 
  in 
  Pteronura, 
  but 
  as 
  short, 
  lateral, 
  integumentary, 
  ridgc-Wcc 
  folds, 
  

   very 
  like 
  limb-folds 
  in 
  normal 
  types, 
  and 
  very 
  near 
  the 
  tip 
  of 
  the 
  tail 
  of 
  

   the 
  embryo; 
  these 
  folds 
  also 
  gradually 
  grow 
  out 
  as 
  rounded 
  lobes, 
  

   finally 
  becoming 
  acuminate. 
  How 
  little 
  tbe 
  mode 
  of 
  development 
  of 
  

   the 
  cetacean 
  flukes 
  resembles 
  the 
  adult 
  condition 
  of 
  the 
  tail 
  in 
  the 
  

   margined-tailed 
  otter 
  may 
  bo 
  gathered 
  from 
  the 
  following 
  extract, 
  de- 
  

   scriptive 
  of 
  that 
  organ 
  in 
  Pteronura, 
  from 
  a 
  paper 
  by 
  J. 
  E. 
  Gray.* 
  On 
  

   p. 
  G5 
  (I. 
  c. 
  infra) 
  it 
  is 
  stated 
  : 
  Tail 
  conical, 
  tapering, 
  rather 
  depressed, 
  

   covered 
  with 
  short 
  hair 
  and 
  furnished 
  with 
  a 
  subcylindrical 
  prominent 
  

   ridge 
  on 
  each 
  side 
  ; 
  end 
  more 
  depressed, 
  two-edged, 
  and 
  fringed 
  at 
  tip. 
  

  

  I 
  frankly 
  admit, 
  however, 
  that 
  the 
  following 
  question 
  may 
  be 
  very 
  

   pertinently 
  asked 
  by 
  those 
  disposed 
  to 
  dispute 
  the 
  validity 
  of 
  my 
  con- 
  

   clusion 
  that 
  the 
  flukes 
  of 
  Cetacea 
  and 
  Sirenians 
  represent 
  the 
  feet 
  of 
  

   terrestrial 
  mammalia; 
  that 
  is, 
  " 
  Why 
  is 
  it 
  not 
  as 
  reasonable 
  to 
  suppose 
  

   that 
  the 
  flukes 
  may 
  not 
  have 
  arisen 
  as 
  lateral 
  dermal 
  folds 
  in 
  the 
  same 
  

   way 
  that 
  the 
  high, 
  falcate 
  dorsal 
  fin 
  of 
  Orca 
  has 
  been 
  unquestionably 
  

   developed 
  ? 
  " 
  To 
  this 
  1 
  have 
  already 
  in 
  part 
  replied 
  by 
  showing 
  in 
  the 
  

   first 
  place 
  that 
  the 
  dorsal 
  fin 
  of 
  cetaceans 
  is 
  not 
  always 
  developed, 
  and, 
  

   secondly, 
  that 
  when 
  present 
  it 
  is 
  always 
  developed, 
  so 
  far 
  as 
  we 
  know, 
  

   after 
  the 
  flukes 
  have 
  been 
  formed 
  as 
  lateral 
  folds. 
  I 
  may 
  therefore 
  per- 
  

   tinently 
  put 
  the 
  following 
  question 
  for 
  objectors 
  to 
  my 
  hypothesis 
  to 
  

   answer: 
  "Why 
  is 
  it 
  that 
  the 
  flukes 
  are 
  always 
  present, 
  and 
  always 
  in 
  

   a 
  lateral 
  position, 
  or 
  approximately 
  in 
  the 
  place 
  of 
  a 
  limb 
  laterally, 
  and 
  

   why 
  there 
  is 
  not 
  only 
  very 
  often 
  a 
  correlation 
  in 
  size 
  but 
  also 
  of 
  form 
  

   between 
  the 
  fore 
  limb 
  and 
  the 
  flukes 
  of 
  such 
  tyftes 
  as 
  Megaptera 
  and 
  

   Rhachianectes 
  ? 
  " 
  And, 
  finally, 
  why 
  is 
  it 
  that 
  the 
  flukes, 
  as 
  a 
  rule, 
  exceed 
  

   the 
  fore 
  limb 
  in 
  size, 
  if 
  such 
  fact 
  does 
  not 
  indicate 
  that 
  the 
  flukes 
  have 
  

   been 
  derived 
  from 
  the 
  integuments 
  of 
  a 
  pes 
  which 
  has 
  been 
  devel- 
  

   oped 
  in 
  an 
  ancestral 
  form, 
  as 
  in 
  the 
  existing 
  pinnipeds, 
  to 
  dimensions 
  

   generally 
  in 
  excess 
  of 
  those 
  of 
  the 
  man 
  us? 
  Another 
  query 
  it 
  may 
  also 
  

   be 
  interesting 
  for 
  objectors 
  to 
  my 
  hypothesis 
  to 
  settle, 
  viz, 
  the 
  variable 
  

   position 
  of 
  the 
  dorsal 
  fin 
  in 
  respect 
  to 
  the 
  paired 
  appendages 
  of 
  ceta- 
  

   ceans, 
  showing 
  that 
  it 
  grows, 
  as 
  I 
  have 
  asserted, 
  from 
  some 
  part 
  of 
  an 
  

   extended 
  dorsal 
  integumentary 
  fold, 
  as 
  attested 
  by 
  the 
  observed 
  facts 
  

   of 
  development, 
  as 
  well 
  by 
  its 
  extreme 
  anterior 
  position, 
  in 
  Globioceph- 
  

   alus 
  and 
  Orca, 
  and 
  its 
  posterior 
  one 
  in 
  tiibbaldins 
  and 
  Bcrardius. 
  Lateral 
  

   carina 
  arc, 
  on 
  the 
  contrary, 
  never 
  found 
  in. 
  front 
  of 
  the 
  base 
  of 
  the 
  

   flukes 
  of 
  cetacean 
  embryo, 
  these 
  organs 
  always 
  developing 
  from 
  short 
  

   lateral 
  folds 
  at 
  the 
  sides 
  of 
  the 
  end 
  of 
  the 
  tail. 
  Such 
  lateral 
  caudal 
  

   carina 
  are 
  also 
  absent 
  in 
  the 
  adult, 
  judging 
  from 
  the 
  casts 
  in 
  the 
  U. 
  S. 
  

   National 
  Museum. 
  

  

  The 
  caudal 
  median 
  notch 
  posteriorly 
  between 
  the 
  bases 
  of 
  the 
  flukes 
  

   is 
  also 
  developed 
  only 
  after 
  the 
  flukes 
  are 
  fully 
  formed 
  in 
  the 
  embryo, 
  

  

  * 
  Observations 
  on 
  the 
  margined-tailed 
  Otter. 
  (Pteronura 
  sandbachii.) 
  Proc. 
  Zool. 
  

   Soc., 
  London, 
  1868, 
  pp. 
  Gl-GG 
  ; 
  Plate 
  VII. 
  

  

  