AN  EXTINCT  GENUS  OF  THE  STUEIONIDJE. 
883 
importance  to  the  more  remarkable  and  distinctive  characters  which  stamp  the  generic 
discrepancy  of  Chondrosteus  from  all  existing  forms  of  fish*. 
Specific  characters. 
The  collection  of  Fossil  Fishes  in  the  British  Museum  contains  two  specimens  oi  Chon- 
drosteus, showing  some  of  the  head-bones  and  a pectoral  fin  of  individuals  of  gigantic 
proportions.  Mr.  Hareisox,  of  Charmouth,  has  kindly  forwarded  for  inspection  a tail 
of  proportionate  size,  and  which  is  supposed  to  have  belonged  to  the  same  individual  as 
one  of  the  Museum  specimens.  Although  size  alone  is  no  criterion  of  specific  difference, 
it  is  nevertheless  of  some  importance  in  this  instance,  where  out  of  a very  large  number 
of  specimens  found  in  one  locality,  some  are  so  greatly  disproportioned  as  to  be  easily 
eliminated,  while  none  afford  any  evidence  of  gradation  from  large  to  small.  It  is  not, 
however,  necessary  to  rely  upon  size  alone  in  this  case,  since  it  is  associated  with  other 
peculiarities  constituting  specific  characters.  The  only  portions  of  the  head  recognizable 
in  the  specimen  in  the  British  Museum  are  the  parietal  and  mastoid  bones.  The  sur- 
face of  these  is  rough  and  granular,  showing  a considerable  amount  of  dermal  consolida- 
tion. The  outer  posterior  angles  of  the  mastoid  bones  are  extended  backwards,  and 
afford  articular  surfaces  for  the  attachment  of  the  upper  extremities  of  the  suprasca- 
pulars.  One  of  the  latter  bones  is  preserved,  and  measures  6 inches  in  length.  The 
tail,  from  the  collection  of  Mr.  Harrison,  measures  18  inches  in  length.  The  lower 
lobe  is  imperfect,  but  the  total  expanse  of  the  organ  cannot  have  been  less  than 
20  inches.  The  length  of  the  entire  fish  must  probably  have  been  above  5 feet.  The 
most  remarkable  character  of  the  caudal  fin  is  the  excessive  development  of  the  scales 
bordering  the  upper  margm.  The  size  and  solidity  of  these  scales  is  such  that  they 
more  resemble  the  broad  teeth  of  an  Aci'odiis  than  any  dermal  development  with  which 
I am  acquainted.  In  consequence  of  this  pecuharity,  I have  named  this  species  Chon- 
drosteus pachyurus.  The  numerous  specimens  derived  from  smaller  individuals  present 
indications  of  two  species,  one  haHng  the  bones  of  the  head  thin  and  smooth,  the  other 
being  characterized  by  stronger  cranial  plates,  having  a granulated  exterior.  The  anal 
fins  of  the  latter  are  more  massive,  and  have  the  transverse  articulations  at  shorter 
inteiTals  (Plate  LXX.  A).  I propose  to  retain  the  Agassizian  name  Chondrosteus 
acipenseroides  for  the  former,  and  to  designate  the  latter  as  Chondrosteus  crassior. 
General  remarks. — The  Sturgeons  were  considered  by  Cuvier,  in  his  classification 
of  fishes,  to  belong  to  the  great  subdmsion  designated,  in  consequence  of  the  cartila- 
ginous condition  of  the  endoskeleton,  Pisces  cartilaginei ; Professor  Agassiz,  however, 
in  his  great  and  successful  efibrt  to  incorporate  in  one  general  system  both  the  recent 
and  extinct  fishes,  included  them  in  his  newly  constituted  family  termed  Ganoidei,  from 
the  peculiar  characters  of  the  exoskeleton.  The  propriety  of  the  latter  classification 
has  been  fully  substantiated  by  Professor  John  Muller,  who,  after  a most  careful 
* The  gigantic  fossil  fish  of  the  Whitby  Lias,  named  by  Agassiz  Gyrosteus  mirahilis,  belongs  to  the 
Sturionidae,  and  is  nearly  allied  to  Chondrosteus. 
