﻿416 
  ACTINOPTEEYGII. 
  

  

  Genus 
  ASPIDORHYNCHUS, 
  Agassiz. 
  

   [Poiss. 
  Foss. 
  vol. 
  ii. 
  pt. 
  i. 
  1833, 
  p. 
  14.] 
  

  

  Kostrum 
  slender, 
  much 
  produced 
  in 
  advance 
  of 
  mandibular 
  sym- 
  

   physis 
  ; 
  circumorbital 
  plates 
  very 
  small, 
  suborbitals 
  large, 
  and 
  an 
  

   intercalary 
  plate 
  between 
  the 
  latter 
  and 
  the 
  preoperculum 
  ; 
  teeth 
  

   irregular 
  in 
  size, 
  largest 
  on 
  the 
  premaxilla, 
  palatine, 
  and 
  presym- 
  

   physial 
  bone, 
  reduced 
  to 
  a 
  fine 
  granulation 
  on 
  the 
  inner 
  face 
  of 
  the 
  

   ectopterygoid 
  ; 
  branchiostegal 
  rays 
  short 
  and 
  broad, 
  and 
  gular 
  plate 
  

   apparently 
  absent. 
  Vertebral 
  centra 
  annular. 
  Fulcra 
  wanting 
  on 
  

   paired 
  fins, 
  minute 
  on 
  median 
  fins. 
  Pelvic 
  fins 
  situated 
  at 
  about 
  

   the 
  middle 
  of 
  the 
  trunk 
  ; 
  dorsal 
  and 
  anal 
  fins 
  short-based, 
  tri- 
  

   angular, 
  remote 
  and 
  opposed 
  ; 
  caudal 
  fin 
  symmetrically 
  forked. 
  

   Scales 
  robust, 
  smooth 
  or 
  rugose 
  ; 
  in 
  three 
  deepened 
  series 
  on 
  the 
  

   flank 
  of 
  the 
  abdominal 
  region, 
  and 
  the 
  foremost 
  scales 
  of 
  the 
  series 
  

   traversed 
  by 
  the 
  lateral 
  line 
  not 
  deeper 
  than 
  the 
  series 
  below. 
  

  

  The 
  chondrocranium 
  of 
  Aspidorhynchvs 
  must 
  have 
  been 
  consider- 
  

   ably 
  ossified, 
  but 
  its 
  characters 
  are 
  as 
  yet 
  unknown. 
  The 
  cranial 
  

   roof-bones 
  form 
  a 
  continuous 
  shield, 
  and 
  the 
  frontals 
  constitute 
  

   much 
  the 
  largest 
  portion 
  of 
  it 
  ; 
  the 
  whole 
  of 
  the 
  roof 
  is 
  more 
  or 
  

   less 
  rugose 
  externally, 
  and 
  the 
  median 
  suture 
  between 
  the 
  frontals 
  

   is 
  remarkably 
  wavy. 
  The 
  parasphenoid 
  is 
  narrow 
  and 
  delicate, 
  

   parallel 
  with 
  the 
  hinder 
  portion 
  of 
  the 
  cranial 
  roof, 
  and 
  apparently 
  

   destitute 
  of 
  teeth. 
  The 
  elongated 
  though 
  robust 
  vomers 
  meet 
  in 
  

   an 
  acute 
  angle 
  below 
  the 
  mesethmoid, 
  and 
  are 
  firmly 
  fused 
  with 
  

   the 
  latter 
  ; 
  they 
  are 
  likewise 
  toothless, 
  so 
  far 
  as 
  the 
  present 
  writer 
  

   has 
  been 
  able 
  to 
  observe 
  them. 
  The 
  rostrum 
  appears 
  to 
  be 
  simple, 
  

   but 
  is 
  often 
  marked 
  with 
  longitudinal 
  ridges 
  which 
  may 
  readily 
  be 
  

   mistaken 
  for 
  separate 
  elements 
  in 
  crushed 
  specimens. 
  The 
  hyo- 
  

   mandibular 
  is 
  much 
  expanded, 
  with 
  a 
  large 
  process 
  for 
  the 
  support 
  

   of 
  the 
  operculum 
  ; 
  and 
  immediately 
  behind 
  the 
  fan-shaped 
  quadrate 
  

   element 
  there 
  is 
  a 
  small 
  narrow 
  symplectic, 
  widest 
  at 
  its 
  upper 
  end. 
  

   The 
  ectopterygoid 
  is 
  toothless 
  behind, 
  where 
  it 
  exhibits 
  a 
  moderately 
  

   deep 
  expansion, 
  but 
  bears 
  further 
  forwards 
  a 
  closely-set 
  series 
  of 
  

   slender 
  teeth, 
  which 
  gradually 
  increase 
  in 
  size 
  to 
  its 
  anterior 
  ex- 
  

   tremity 
  where 
  it 
  articulates 
  with 
  the 
  palatine. 
  The 
  teeth 
  on 
  the 
  

   latter 
  element 
  are 
  largest 
  behind 
  and 
  diminish 
  forwards. 
  The 
  

   entopterygoid 
  is 
  long 
  and 
  narrow, 
  while 
  both 
  this 
  and 
  the 
  im- 
  

   perfectly-known 
  metapterygoid 
  are 
  delicate. 
  The 
  epihyal 
  is 
  small 
  

   and 
  the 
  ceratohyal 
  very 
  large, 
  but 
  the 
  basihyal 
  is 
  unknown. 
  The 
  

   maxilla 
  is 
  much 
  elongated 
  and 
  slender, 
  bearing 
  a 
  series 
  of 
  relatively 
  

   small 
  teeth 
  ; 
  and 
  there 
  is 
  a 
  curious 
  laminar 
  expansion 
  along 
  its 
  

   upper 
  border 
  immediately 
  in 
  advance 
  of 
  the 
  orbit. 
  There 
  are 
  also 
  

  

  