﻿INTKODTJCTIOIT. 
  xvii 
  

  

  Saurojpsis, 
  apparently 
  ranging 
  from 
  the 
  Upper 
  Lias 
  to 
  the 
  Lower 
  

   Kimmeridgian, 
  and 
  Euihynotus, 
  confined 
  to 
  the 
  Upper 
  Lias, 
  are 
  the 
  

   least 
  specialized 
  genera 
  of 
  Pachycormidae. 
  Pachycormus 
  itself 
  is 
  

   known 
  only 
  from 
  the 
  Upper 
  Lias, 
  and 
  is 
  a 
  little 
  more 
  modified, 
  

   having 
  lost 
  the 
  pelvic 
  fins. 
  Hypsocormus, 
  from 
  the 
  Oxfordian 
  and 
  

   Kimmeridgian, 
  exhibits 
  more 
  powerful 
  teeth 
  than 
  any 
  of 
  the 
  earlier 
  

   genera, 
  these 
  fixed 
  to 
  the 
  base 
  of 
  sockets 
  which 
  are 
  incomplete 
  on 
  

   the 
  inner 
  side 
  ; 
  and 
  there 
  is 
  one 
  species, 
  H. 
  leedsi, 
  possessing 
  a 
  

   slightly 
  more 
  prominent 
  rostrum 
  than 
  any 
  earlier 
  fish. 
  The 
  latest 
  

   genus 
  Protosphyrcena, 
  known 
  only 
  by 
  comparatively 
  fragmentary 
  

   specimens 
  from 
  the 
  Cretaceous 
  both 
  of 
  Europe 
  and 
  America, 
  

   nearly 
  always 
  exhibits 
  a 
  much 
  more 
  produced 
  rostrum, 
  while 
  its 
  

   large 
  teeth 
  are 
  fixed 
  in 
  complete 
  sockets 
  ; 
  and 
  the 
  pectoral 
  fins 
  of 
  

   this 
  fish 
  are 
  still 
  narrower 
  and 
  more 
  elongated 
  than 
  those 
  of 
  its 
  

   forerunners 
  in 
  the 
  Jurassic 
  period. 
  

  

  If 
  the 
  interpretation 
  of 
  the 
  Pachycormidae 
  suggested 
  in 
  this 
  

   Catalogue 
  prove 
  correct, 
  there 
  is 
  thus 
  most 
  distinct 
  evidence 
  of 
  

   gradual 
  progression 
  as 
  they 
  are 
  traced 
  upwards 
  in 
  their 
  range. 
  

  

  Aetheospondyli. 
  

  

  The 
  recognition 
  of 
  this 
  group 
  is 
  a 
  confession 
  of 
  ignorance. 
  As 
  to 
  

   the 
  origin 
  and 
  immediate 
  relationships 
  of 
  the 
  Aspidorhynchidae 
  

   and 
  Lepidosteidae, 
  there 
  is 
  still 
  not 
  the 
  faintest 
  clue. 
  That 
  these 
  

   two 
  families 
  are 
  close 
  allies 
  is 
  generally 
  admitted 
  ; 
  that 
  they 
  are 
  

   connected 
  in 
  some 
  way 
  with 
  the 
  Protospondyli 
  as 
  above 
  defined, 
  is 
  

   also 
  a 
  common 
  belief. 
  They 
  are 
  peculiar, 
  however, 
  in 
  exhibiting 
  

   annular 
  or 
  solid 
  centra 
  throughout 
  the 
  length 
  of 
  the 
  vertebral 
  

   column, 
  no 
  alternating 
  pleurocentra 
  and 
  hypocentra 
  even 
  in 
  the 
  

   caudal 
  region 
  ; 
  and 
  they 
  cannot 
  be 
  referred 
  to 
  the 
  Isospondyli 
  on 
  

   account 
  of 
  the 
  complexity 
  of 
  the 
  mandible. 
  They 
  may 
  thus 
  be 
  pro- 
  

   visionally 
  assigned 
  to 
  an 
  intermediate, 
  though 
  scarcely 
  annectant 
  

   group, 
  which 
  is 
  named 
  Aetheospondyli 
  in 
  allusion 
  to 
  the 
  unique 
  

   variations 
  of 
  the 
  vertebral 
  centra 
  from 
  the 
  biconcave 
  or 
  amphi- 
  

   ccelous 
  to 
  an 
  opisthoccelous 
  type. 
  

  

  AspidorhyncMdce. 
  

  

  The 
  Aspidorhynchidae 
  are 
  long-bodied 
  fishes 
  with 
  deep 
  ganoid 
  

   flank-scales, 
  small 
  fins, 
  and 
  elongated 
  snout, 
  ranging 
  from 
  the 
  

   Bathonian 
  to 
  the 
  Upper 
  Cretaceous 
  inclusive. 
  They 
  are 
  remarkable 
  

   for 
  the 
  possession 
  of 
  a 
  median 
  presymphysial 
  bone 
  in 
  the 
  mandible. 
  

  

  past 
  ni. 
  b 
  

  

  