﻿LEPTOLEPIBJi. 
  521 
  

  

  Genus 
  THRISSOPS, 
  Agassiz. 
  

   [Poiss. 
  Foss. 
  vol. 
  ii. 
  pt. 
  i. 
  1833, 
  p. 
  12.] 
  

  

  Head 
  small 
  and 
  teeth 
  minute 
  ; 
  sclerotic 
  ossified. 
  Maxilla 
  arched, 
  

   with 
  a 
  slightly 
  convex 
  dentigerous 
  border; 
  mandible 
  prominent, 
  

   the 
  dentary 
  apparently 
  intermediate 
  in 
  form 
  between 
  that 
  of 
  

   Leptolepis 
  and 
  Aetlialion. 
  Opercular 
  apparatus 
  as 
  in 
  Leptolepis. 
  

   Vertebral 
  centra 
  well 
  ossified, 
  strengthened 
  by 
  a 
  median 
  lateral 
  

   ridge 
  ; 
  the 
  free 
  neural 
  arches 
  in 
  the 
  abdominal 
  region 
  much 
  elon- 
  

   gated 
  and 
  thickened, 
  the 
  ribs 
  also 
  especially 
  robust. 
  Pelvic 
  fins 
  

   much 
  smaller 
  than 
  the 
  pectorals 
  ; 
  dorsal 
  fin 
  small 
  and 
  short-based, 
  

   opposite 
  to 
  the 
  anal 
  fin, 
  which 
  is 
  acuminate 
  in 
  front 
  and 
  much 
  

   extended 
  behind 
  ; 
  caudal 
  fin 
  forked. 
  Scales 
  completely 
  covering 
  

   the 
  trunk 
  ; 
  no 
  enlarged 
  or 
  thickened 
  ridge-scales. 
  

  

  Thrissops 
  formosus, 
  Agassiz. 
  

  

  1833-44. 
  Thrissop>s 
  formosus, 
  L. 
  Agassiz, 
  Poiss. 
  Foss. 
  vol. 
  ii. 
  pt. 
  i. 
  p. 
  12 
  ; 
  

  

  pt. 
  ii. 
  p. 
  124, 
  pi. 
  lxv. 
  a. 
  

   1844. 
  Thrissops 
  subovatus, 
  L. 
  Agassiz 
  (ex 
  Minister, 
  MS.), 
  ibid. 
  pt. 
  ii. 
  

  

  p. 
  128 
  (undefined). 
  [Fish 
  ; 
  Palseontological 
  Museum, 
  Munich.] 
  

   1839. 
  Thrissops 
  ovatus, 
  G. 
  von 
  Minister, 
  Neues 
  Jahrb. 
  p. 
  680. 
  . 
  [Fish 
  ; 
  

  

  Palseontological 
  Museum, 
  Munich.] 
  

   1852. 
  Thrissops 
  formosus, 
  F. 
  A. 
  Quenstedt, 
  Handb. 
  Petrefakt. 
  p. 
  219, 
  

  

  pi. 
  xvii. 
  fig. 
  19. 
  

   1863. 
  Thrisso2)sfor?nosus,A.V?&gner, 
  Abh. 
  k. 
  bay. 
  Akad. 
  Wiss., 
  math.- 
  

  

  phys. 
  CI. 
  vol. 
  ix. 
  p. 
  734. 
  

   1863. 
  TJwissops 
  subovatus, 
  A. 
  Wagner, 
  ibid. 
  p. 
  734. 
  

   1888. 
  Thrissops 
  formosus, 
  K. 
  A. 
  von 
  Zittel, 
  Handb. 
  Palseont. 
  vol. 
  hi. 
  

  

  p. 
  273, 
  figs. 
  280, 
  281. 
  

  

  Type. 
  Nearly 
  complete 
  fish 
  ; 
  Palseontological 
  Museum, 
  Munich. 
  

  

  The 
  type 
  species, 
  attaining 
  a 
  length 
  of 
  about 
  0*5. 
  Head 
  with 
  

   opercular 
  apparatus 
  occupying 
  from 
  one-eighth 
  to 
  one-seventh 
  of 
  

   the 
  total 
  length 
  of 
  the 
  fish 
  ; 
  maximum 
  depth 
  of 
  trunk 
  much 
  

   exceeding 
  the 
  length 
  of 
  the 
  head 
  with 
  opercular 
  apparatus, 
  and 
  

   contained 
  about 
  three-and-a-half 
  times 
  in 
  the 
  length 
  of 
  the 
  trunk 
  

   from 
  the 
  pectoral 
  arch 
  to 
  the 
  base 
  of 
  the 
  caudal 
  fin. 
  Vertebras 
  at 
  

   least 
  60 
  in 
  number, 
  about 
  32 
  being 
  abdominal 
  and 
  2S 
  caudal. 
  

   Pelvic 
  fins 
  arising 
  much 
  nearer 
  to 
  the 
  anal 
  tban 
  to 
  the 
  pectorals 
  

   and 
  comprising 
  about 
  5 
  rays 
  ; 
  dorsal 
  fin, 
  with 
  14 
  rays, 
  arising 
  

   somewhat 
  behind 
  the 
  origin 
  of 
  the 
  anal, 
  which 
  is 
  much 
  elevated 
  

   in 
  front 
  and 
  comprises 
  not 
  less 
  than 
  30 
  rays 
  ; 
  caudal 
  fin 
  very 
  deeply 
  

   forked 
  and 
  the 
  lobes 
  slender. 
  Scales 
  partly 
  ornamented 
  with 
  

   delicate 
  radiating 
  strise. 
  

  

  