AN  EXTINCT  GENUS  OE  THE  STUEIONID^. 
in  a position  which  leads  to  the  inference  that  it  may  be  the  posterior  portion  of  this 
bone.  It  proceeds  from  underneath  the  premaxillary,  and  has  an  upward  process  over- 
lapping the  palatine  bone,  and  a downward  process  extending  in  the  direction  of  the  arti- 
culation of  the  lower  mandible*.  The  lower  jaw  (Plate  LXIX.  32)  is  composed  of  a pair 
of  single  bones,  narrow  in  front,  and  gradually  expanding  as  they  recede  from  the  sym- 
physial  suture.  Their  upper  margins  are  edentulous.  Their  outer  surface  is  corru- 
gated, like  that  of  the  premaxillary  bones,  the  rugae  being  longitudinal.  In  a depression 
between  two  of  the  central  rugae,  a series  of  minute  apertures  are  discernible,  for  the 
passage  of  the  vessels. 
The  maxillary  and  mandibular  arches  are  suspended,  as  in  the  recent  Sturgeon,  from 
the  tympanic  pedicle.  Of  the  bones  and  cartilages  constituting  this  apparatus,  the  epi- 
tympanic  bone  (Plate  LXIX.  28  a)  is  the  part  most  easily  identified,  and  most  constantly 
preserved.  It  is  seen  sloping  backwards  and  downwards  from  a point  corresponding 
with  the  mastoid  process  of  bony  fishes.  The  articulating  process  and  the  upper  third 
of  the  shaft  are  rounded ; the  lower  extremity  is  expanded  into  a thin  and  broad  plate. 
Compared  with  the  epitympanic  bone  of  a recent  Sturgeon,  it  differs  in  being  longer  and 
thinner.  The  upper  extremity  is  smaller  and  the  lower  broader,  and  more  triangular 
in  outline,  and  the  entire  bone  constitutes  a far  larger  portion  of  the  arch.  The  suc- 
ceeding elements,  namely,  the  mesotympanic  and  hypotympanic,  which  are  represented 
in  the  Sturgeon  by  two  cartilages,  are  united  to  form  one  bony  plate  (Plate  LXIX.  28  d), 
somewhat  crescentic  in  form  and  having  a tuberosity  on  the  interior  face,  at  a point 
corresponding  with  the  junction  of  the  two  parts,  for  the  attachment  of  the  hyoidean 
arch.  This  compound  b’one  is  small  and  feeble  compared  with  the  great  strength  and 
expanse  of  the  epitympanic.  The  upper  extremity  which  meets  the  anterior  angle  of 
the  spatulate  termination  of  the  epitympanic  is  thin  and  pointed,  and  ill-adapted,  either 
in  form  or  dimensions,  for  affording  an  articulation  of  sufficient  strength  for  the  support 
of  the  mandibular  and  maxillary  organs.  It  probably  represents  merely  the  thick 
anterior  column  of  cartilage,  which  in  the  recent  Sturgeon  is  of  great  strength  and 
solidity;  the  remainder  of  the  attachment  being  completed  by  a plate  of  cartilage. 
The  hypotjunpanic  portion  of  the  bone  is  also  thin,  but  rather  stronger  than  the  upper- 
half.  It  meets  the  condyloid  extremity  of  the  lower  jaw  at  an  open  angle.  This 
joint,  as  w-ell  as  the  connexion  with  the  palatine  bone,  was  probably  completed  by 
cartilages,  as  in  the  recent  tyjre. 
The  dislocated  condition  of  the  specimens  renders  it  difficult  to  determine  the  consti- 
tuent parts  of  the  hyoid  arch.  In  two  instances,  a single,  broad  umbonated  bone 
(Plate  LXVIII.  fig.  2,  42)  is  seen  behind  and  below  the  symphysis  of  the  lower  jaw, 
which  must  be  considered  either  as  composed  of  the  two  basihyals  united  to  form 
a keystone  of  the  arch,  or  as  the  homologue  of  the  great  glossohyal  or  lingual  bone 
found  in  the  recent  genera  Ay'apaima,  Elops  and  Ainia,  and  in  the  extinct  genus 
LopMostemus.  A bone  (Plate  LXVIII.  fig.  2, 4o),  which  probably  represents  one  of  the 
* It  is  not  improbable  that  this  may  prove  to  be  the  premaxillary,  and  that  described  as  the  premaxillary, 
the  maxillary  bone. 
