AN  EXTINCT  GENUS  OF  THE  STUEIONID^. 
873 
fin  is  placed  above  the  anal  fin,  having  its  anterior  rays  a little  in  advance  of  the  first 
mys  of  that  fin.  In  Chondrosteus  the  ventral  fins  are  found  midway  between  the  pec- 
toral fins  and  the  tail ; the  dorsal  fin  is  immediately  above  the  ventral  fins,  and  the  anal 
fin  approximated  to  the  lower  lobe  of  the  tail.  As  regards  size,  all  the  fins  are  pro- 
portionately larger  than  those  of  the  existing  Sturgeons,  as  might  naturally  be  inferred 
from  the  greater  bulk  of  the  fish.  In  proceeding  to  give  a detailed  description  of  the 
organs  of  locomotion  and  their  associated  parts,  I commence  with  the  bony  framework 
which  afibrds  attachment  and  support  to  the  pectoral  fins;  premising  that  the  letters 
and  numbers  by  which  the  several  parts  are  designated  on  the  drawings  are  those 
employed  by  Professor  Owen  in  his  tabular  notation  of  the  homologies  of  the  Vertebrate 
Skeleton. 
Scapular  Arch  and  its  Appendages. — In  the  recent  Sturgeons  the  strong  bony  girdle 
encircling  the  thoracic  region  of  the  trunk  and  providing  a firm  attachment  for  the 
large  pectoral  fins,  is  composed  of  three  elements  on  either  side,  the  suprascapular,  the 
scapular,  and  the  coracoid.  In  the  extinct  genus  this  number  is  reduced  to  two  by  the 
coalescence  of  the  two  latter  to  form  a single  bone  (Plates  LX VIII.  fig.  I,  and  LXIX.  si,  52). 
This  scapulocoracoid  bone,  fi’om  its  size  and  comparative  hardness,  is  more  frequently 
preserved  and  more  perfectly  displayed  than  any  other  of  the  constituent  parts  of  the 
cranial  framework.  The  external  portion  is  crescentic,  the  concavity  being  anterior  for 
the  reception  of  the  posterior  margin  of  the  opercular  flap.  At  the  point  of  attachment 
of  the  pectoral  fin  the  bone  is  thick  and  rounded;  the  upper  or  scapular  portion  expands 
gradually  into  a triangular  plate,  thinning  off  towards  the  upper  extremity  for  adjust- 
ment with  the  suprascapular  bone ; the  lower  or  coracoid  portion  also  expands  as  it 
descends,  sweeping  under  the  thorax  and  meeting  the  corresponding  bone  of  the  oppo- 
site side  on  the  median  line.  The  exposed  surfaces  of  these  bones  are  prettily  orna- 
mented with  sharp  undulating  asperities,  not  unlike  the  surface  ornament  of  the  shell  of 
Pholas  ci'ispata.  Anteriorly  each  scapulocoracoid  expands  into  a broad  concave  plate, 
dhected  inwards  and  forming  the  platform  of  the  branchial  cavity,  and  partitioning  it 
off  from  the  thoracic  cavity.  This  portion  of  the  bone  has  the  surface  perfectly  smooth. 
The  upper  or  suprascapular’  member  of  the  arch  (Plate  LXIX.  50)  overlaps  the  extre- 
mity of  the  scapulocoracoid  bone,  and  extends  thence  to  the  occipital  region  of  the  skull. 
It  is  of  slender  dimensions  compared  with  the  homologous  bone  of  the  recent  Sturgeons. 
The  ornamentation  characteristic  of  the  external  portion  of  the  scapulocoracoid  bones 
is  continued  along  the  anterior  margin  of  the  suprascapular,  but  the  greater  portion 
of  the  bone  seems  to  have  been  invested  with  the  dermal  integument;  in  this  respect 
corresponding  with  the  homologous  bone  in  the  ordinary  ganoid  fishes.  The  pectoral 
fins  (Plates  LXVII.,  LXVIII.  fig.  I,  and  LXIX.  P)  are  attached  to  the  scapulocoracoids 
immediately  behind  the  posterior  angles  of  these  bones.  They  are  proportionately  much 
broader  and  more  rounded  at  their  extremities  than  these  organs  of  the  recent  Sturgeon, 
but  the  anterior  ray  is  not  nearly  so  strong.  They  have  no  representatives  of  radius,  ulna, 
or  carpus,  but  the  fin-rays  are  articulated  to  a series  of  strong  metacarpal  bones,  of  which 
5x2 
