﻿INTRODUCTION. 
  XI 
  

  

  Here, 
  again, 
  the 
  earliest 
  term 
  in 
  the 
  series 
  (Ophiopsis) 
  is 
  the 
  

   most 
  generalized 
  ; 
  and 
  it 
  has 
  the 
  most 
  extensive 
  range 
  (Upper 
  

   Trias 
  — 
  Purbeckian). 
  It 
  is, 
  indeed, 
  a 
  distinct 
  link 
  between 
  the 
  

   family 
  of 
  Macrosemiidae 
  (to 
  which 
  this 
  Catalogue 
  assigns 
  it) 
  and 
  

   that 
  of 
  the 
  Eugnathidae 
  to 
  be 
  considered 
  below. 
  The 
  Lower 
  Kim- 
  

   meridgian 
  and 
  Purbeckian 
  species 
  are 
  also 
  interesting 
  as 
  exhibiting 
  

   ring-vertebrae, 
  the 
  separate, 
  alternating 
  pleurocentral 
  and 
  hypo- 
  

   central 
  rings 
  being 
  sometimes 
  observable 
  in 
  part 
  of 
  the 
  caudal 
  

   region. 
  

  

  The 
  diminutive 
  Rhaetic 
  Legnonotus, 
  with 
  greatly 
  extended 
  dorsal 
  

   fin, 
  is 
  a 
  more 
  pronounced 
  member 
  of 
  the 
  Macrosemiidae. 
  This 
  has 
  

   an 
  apparently 
  regular 
  squamation 
  ; 
  but 
  the 
  closely-related 
  genus 
  

   Macrosemius 
  (Lower 
  Kimmeridgian, 
  Portlandian, 
  and 
  Purbeckian) 
  

   is 
  remarkable 
  for 
  the 
  degeneration 
  of 
  its 
  scales, 
  which 
  are 
  very 
  thin 
  

   and 
  subdivide 
  towards 
  the 
  dorsal 
  and 
  ventral 
  margins, 
  being 
  usually 
  

   wanting 
  near 
  the 
  former. 
  The 
  dwarfed 
  species 
  of 
  Portlandian 
  and 
  

   Purbeckian 
  age 
  may 
  perhaps 
  have 
  been 
  entirely 
  scaleless. 
  There 
  are 
  

   no 
  fulcra 
  except 
  on 
  the 
  caudal 
  fin 
  ; 
  but 
  the 
  rays 
  of 
  the 
  dorsal 
  fin 
  

   in 
  some 
  species 
  are 
  curiously 
  denticulated 
  on 
  the 
  hinder 
  border. 
  

   Traces 
  of 
  vertebral 
  centra 
  are 
  never 
  observable. 
  

  

  Histionotus 
  is 
  a 
  thick-scaled 
  contemporary 
  of 
  Macrosemius, 
  with 
  

   fulcra 
  on 
  all 
  the 
  fins 
  and 
  with 
  delicate 
  ring- 
  vertebrae. 
  Propterus 
  

   and 
  Notagogus, 
  of 
  the 
  same 
  age, 
  are 
  characterized 
  by 
  the 
  subdivision 
  

   of 
  the 
  dorsal 
  fin 
  into 
  two 
  parts 
  ; 
  and 
  the 
  former 
  has 
  thinner 
  scales 
  

   and 
  more 
  delicate 
  vertebrae 
  than 
  the 
  latter. 
  

  

  The 
  late 
  Cretaceous 
  genus 
  Petalopteryx 
  is 
  an 
  excessively 
  elon- 
  

   gated 
  fish 
  with 
  irregularly 
  arranged 
  rhombic 
  scales, 
  and 
  the 
  robust 
  

   cheek-plates 
  subdivided 
  into 
  small 
  tesserae. 
  

  

  The 
  two 
  most 
  characteristic 
  features 
  of 
  the 
  Macrosemiidae 
  — 
  the 
  

   elongated 
  trunk 
  and 
  excessively 
  extended 
  dorsal 
  fin 
  — 
  are 
  thus 
  

   developed 
  early 
  in 
  a 
  Rhaetic 
  fish 
  with 
  normal 
  squamation 
  (Legno- 
  

   notus)-, 
  ring-vertebrae 
  are 
  only 
  well-formed 
  in 
  the 
  genera 
  which 
  

   retain 
  comparatively 
  thick 
  scales 
  ; 
  the 
  scales 
  and 
  fulcra 
  begin 
  to 
  

   degenerate 
  in 
  the 
  Upper 
  Jurassic 
  (Macrosemius) 
  ; 
  the 
  scales, 
  though 
  

   still 
  rhombic, 
  are 
  irregularly 
  subdivided 
  in 
  the 
  Cretaceous 
  Petalo- 
  

   pterycc, 
  and 
  in 
  this 
  unique 
  genus 
  the 
  cheek-plates 
  are 
  represented 
  

   by 
  an 
  investment 
  of 
  small 
  tesserae. 
  

  

  Pycnodontidce. 
  

  

  A 
  curious 
  parallelism 
  will 
  be 
  noticed 
  in 
  the 
  development 
  of 
  the 
  

   Semionotidae 
  and 
  Macrosemiidae 
  as 
  thus 
  briefly 
  sketched, 
  Vertebral 
  

  

  