882 
SIE  P.  G.  EGEETON  ON  CHONDEOSTEUS, 
edge  is  straight,  and  gives  attachment  to  a small  subopercular  plate  (Plates  LX'\~II. 
and  LXIX.  se),  having  a still  smaller  interoperculum  (Plates  LXVII.  and  LXIX.  37) 
fitted  to  its  anterior  m^argin.  These  are  succeeded  below  by  the  branchiostegous  rays 
described  in  a former  part  of  this  memoir.  This  arrangement  of  the.  opercular  appa- 
ratus corresponds  with  that  of  some  of  the  extinct  fossil  fishes.  The  preoperculum  is 
wanting.  The  external  characters  of  the  cranial  envelope  tend  to  the  conclusion  that 
the  dermal  crust  was  very  feebly  developed  in  this  region,  and  lead  to  the  further 
inference  that  a proportionate  deficiency  would  be  found  in  other  parts  of  the  tegu- 
nreirtary  covering.  This  was  probably  the  case,  for  with  the  exception  of  the  scales 
investiirg  the  caudal  fin,  no  trace  has  beeir  discovered  of  the  dermal  plates,  so  charac- 
teristic of  the  recent  Sturgeon.  The  negative  evidence  of  the  non-existence  of  this 
peculiar  feature  is  strengthened  by  the  fact,  that  of  all  the  component  tissues  these 
dermal  plates  are  the  most  indestructible,  and  would  therefore  have  resisted  decompo- 
sitiorr  more  effectually  tharr  other  more  tender  parts,  which  are  nevertheless  found  in 
the  fossil  state.  The  broad  nuchal  scale,  for  instance,  so  prominent  in  the  recent 
Sturgeon,  would  have  left  some  trace  of  its  existence  in  specimens,  where  not  only  the 
surrounding  bones,  but  even  the  semicartilaginous  remains  of  the  subjacent  vertebral 
neural  arches,  are  distinctly  preserved. 
Another  feature  bearing  upon  this  question  is  seen  in  the  natatory  organs.  In  the 
recent  Sturgeon,  the  dorsal,  anal,  and  caudal  fins  are  each  provided  with  a short  anterior 
spine  springing  from  a dermal  plate,  whereas  in  the  fossil  species  no  such  spine-bearing 
scales  occur,  the  first  fin-rays  being  supported  by  interapophysial  osselets,  as  in  other 
bony  fishes.  In  orre  specimerr  examined  by  me,  a portion  of  the  skin  is  preserwed. 
Viewed  under  the  microscope,  all  structure  characteristic  of  dermal  tissrre  is  obliterated, 
arrd  the  substarrce  resembles  a thirr  layer  of  carbonaceous  material,  perfectly  homoge- 
neorrs,  arrd  showirrg  no  trace  of  the  borry  spiculte,  which  to  a greater  or  less  exterrt  beset 
the  smooth  portiorrs  of  the  skin  in  the  recent  Sturgeon.  I am  therefore  inclirred  to 
thirrk  that  the  fossil  Sturgeorr  was  smooth  and  scaleless,  like  the  Spatularise  of  the  Xorth 
American  rivers,  although  in  other  details  of  orgarrizatiorr  it  had  greater  affinity  to  the 
more  typical  Sturgeons.  Of  these,  the  Sca^hyrhynchi  have  the  head-bones  almost  as 
smooth  as  those  of  the  Polyodon,  but  the  development  of  the  dermal  scales  orr  the  trrrnk 
is  excessive,  the  caudal  regiorr  being  wholly  errcased  with  defensive  armour.  Irr  some 
species  we  see  the  dermal  layer  thick  and  rugged  orr  the  head,  and  yet  feebly  developed 
irr  the  trurrk ; while  in  others  the  deposition  of  bony  material  is  coexterrsive  orr  both  the 
head  arrd  body.  With  regard  to  form,  some  are  long  and  slerrder,  others  are  more 
massive ; some  have  elongated  snouts,  others  have  the  rostrurrr  more  abrupt. 
Of  all  the  species  I have  examined,  Adpenser  Guldmstddti  has  the  rrearest  resem- 
blarrce  to  the  fossil  genus.  The  short  snout,  the  proportiorrs  of  the  crarrial  plates,  arrd 
the  depth  of  the  arrterior  part  of  the  body,  are  to  a certain  extent  approaches  to  the 
corrdition  of  these  parts  in  the  extirrct  form,  and  they  are  combirred  with  the  rrrirrirnurrr 
amourrt  of  calcification  of  the  integument ; at  the  same  time  these  are  features  of  rrriiror 
