﻿[49] 
  DEVELOPMENT 
  OF 
  THE 
  CETACEA. 
  475 
  

  

  tomes 
  over 
  12 
  to 
  14 
  vertebr.se 
  are 
  obliterated 
  and 
  converted 
  into 
  tendon 
  

   posteriorly, 
  so 
  as 
  to 
  call 
  for 
  a 
  new 
  mode 
  of 
  distribution 
  of 
  the 
  nerves 
  

   different 
  from 
  that 
  which 
  obtains 
  in 
  fishes 
  in 
  which 
  the 
  paired 
  nerves 
  

   and 
  muscular 
  segments 
  correspond 
  almost 
  exactly 
  in 
  number 
  with 
  the 
  

   vertebral 
  segments. 
  

  

  VII. 
  — 
  Translocation 
  of 
  the 
  distal 
  ends 
  of 
  the 
  hind 
  limbs 
  

  

  IN 
  THE 
  SIRENIANS. 
  

  

  The 
  foetus 
  of 
  Halicore 
  dugong, 
  figured 
  by 
  Harting,* 
  about 
  11 
  inches 
  

   long, 
  Fig. 
  22, 
  Plate 
  III, 
  shows 
  the 
  flukes 
  well 
  developed 
  and 
  of 
  much 
  

   the 
  same 
  form 
  as 
  in 
  cetaceans. 
  Judging 
  from 
  the 
  permanent 
  adult 
  

   form 
  of 
  the 
  tail 
  of 
  Manatus 
  (outer 
  outline, 
  Fig. 
  20, 
  Plate 
  III), 
  which 
  

   has 
  the 
  most 
  rudimentary 
  type 
  of 
  fluke, 
  found 
  either 
  amongst 
  ceta- 
  

   ceans 
  or 
  sirenians, 
  it 
  is 
  probable 
  that 
  the 
  flukes, 
  in 
  those 
  types 
  having 
  

   them 
  well 
  developed, 
  viz, 
  Halicore 
  and 
  Rhythm, 
  grew 
  out 
  as 
  in 
  the 
  for- 
  

   mer 
  as 
  low 
  lateral 
  horizontal 
  folds. 
  It 
  seems 
  that 
  in 
  Manatee 
  the 
  flukes 
  

   have 
  been 
  arrested 
  in 
  development 
  so 
  that 
  they 
  simulate 
  somewhat 
  the 
  

   early 
  stages 
  of 
  the 
  outgrowth 
  of 
  the 
  cetacean 
  flukes, 
  as 
  shown 
  in 
  the 
  

   accompanying 
  figures 
  of 
  embryos 
  of 
  the 
  latter. 
  

  

  In 
  those 
  fossil 
  forms 
  which 
  are 
  less 
  degenerate 
  than 
  the 
  existing 
  

   species, 
  Halitherium 
  Schinzi, 
  for 
  example, 
  had 
  the 
  rudimentary 
  femur 
  

   directed 
  backwards 
  towards 
  the 
  flukes 
  just 
  as 
  in 
  the 
  tibia 
  in 
  the 
  exist- 
  

   ing 
  cetaceans 
  and 
  pinnipeds, 
  according 
  to 
  the 
  interpretations 
  of 
  Lep- 
  

   sius,t 
  who 
  has 
  given 
  excellent 
  figures 
  of 
  the 
  skeleton 
  of 
  this 
  type. 
  

   This 
  direction 
  of 
  the 
  femur, 
  as 
  already 
  urged 
  in 
  the 
  case 
  of 
  analogously 
  

   modified 
  forms, 
  is 
  very 
  significant, 
  and 
  goes 
  a 
  great 
  way 
  in 
  helping 
  to 
  

   substantiate 
  the 
  view 
  that 
  the 
  flukes 
  are 
  also 
  modified 
  hind 
  limbs 
  in 
  • 
  

   the 
  sirenians. 
  

  

  In 
  Halitherium 
  there 
  is 
  a 
  well-developed 
  acetabular 
  fossa 
  developed 
  

   on 
  the 
  pelvic 
  bones 
  for 
  the 
  reception 
  of 
  the 
  head 
  of 
  the 
  femur. 
  Neither 
  

   femur 
  nor 
  acetabulum 
  is 
  developed 
  in 
  the 
  living 
  genera 
  Halicore 
  and 
  

   Manatee. 
  Phytina 
  probably 
  had 
  the 
  pelvic 
  bones 
  as 
  well 
  developed 
  as 
  

   in 
  Halicore, 
  in 
  which 
  they 
  are 
  present 
  as 
  two 
  pieces, 
  an 
  anterior 
  prob- 
  

   ably 
  corresponding 
  to 
  the 
  ilium 
  and 
  ischium 
  of 
  normal 
  mammals. 
  The 
  

   pelvis 
  in 
  Manatee 
  seems 
  to 
  be 
  composed 
  of 
  a 
  single 
  almost 
  quadrate 
  

   element, 
  as 
  seen 
  from 
  the 
  side, 
  and 
  is 
  so 
  reduced 
  that 
  it 
  represents 
  the 
  

   extremest 
  condition 
  of 
  atrophy 
  of 
  the 
  pelvic 
  elements 
  yet 
  known, 
  unless, 
  

   as 
  Mr. 
  F. 
  W. 
  True 
  thinks, 
  after 
  an 
  unsuccessful 
  search 
  for 
  this 
  element, 
  

   it 
  is 
  altogether 
  absent 
  in 
  Kogia, 
  the 
  pygmy 
  sperm 
  whale. 
  

  

  This 
  condition 
  of 
  degeneracy 
  of 
  the 
  pelvis 
  of 
  sirenians 
  is 
  manifested 
  

  

  * 
  Description 
  de 
  l'ceuf 
  et 
  du 
  placenta 
  de 
  Halicore 
  dugong, 
  suivie 
  de 
  considerations 
  

   6ur 
  le 
  valeur 
  taxonomique 
  et 
  phylogeuique 
  des 
  caracteres 
  differentiels, 
  fournis 
  par 
  le 
  

   placenta 
  des 
  mammiferes. 
  Tijdschrift 
  der 
  Nederlandsche 
  Dierk. 
  Vereen, 
  Dl. 
  IV, 
  1879, 
  

   pp. 
  1-29, 
  pis. 
  I-II. 
  

  

  t 
  Halitherium 
  Schinzi, 
  die 
  fossile 
  Sirene 
  des 
  Mainzer 
  Beckens. 
  Abhand., 
  des 
  Mit- 
  

   telrheinischen 
  geolog. 
  Vereins, 
  I, 
  Lieferungen 
  1 
  and 
  2, 
  4to 
  ; 
  Darmstadt, 
  1831 
  and 
  1882. 
  

  

  