﻿VERTEBRATES. 
  197 
  

  

  PSAMMODONTID^, 
  

  

  L. 
  cle 
  Koninck. 
  

  

  Genus 
  PSAMMODUS, 
  Agassiz. 
  

  

  Psammodus, 
  Agassiz, 
  1843, 
  Poisspns 
  Fossiles, 
  tome 
  III, 
  p. 
  110. 
  

  

  Teeth 
  quadrilateral 
  or 
  trapezoidal 
  in 
  general 
  outline, 
  variable, 
  usu- 
  

   ally 
  thick 
  and 
  massive. 
  The 
  coronal 
  region 
  presents 
  a 
  more 
  or 
  less 
  

   plane 
  surface, 
  according 
  to 
  the 
  position 
  the 
  form 
  occupied 
  upon 
  the 
  

   jaws, 
  always 
  arched, 
  generally 
  moderately, 
  in 
  the 
  longitudinal 
  direc- 
  

   tion 
  or 
  from 
  behind 
  forward, 
  transversely 
  concave 
  (maxillary 
  teeth), 
  

   or 
  more 
  or 
  less 
  convex 
  (mandibular 
  teeth), 
  sometimes 
  raised 
  into 
  a 
  

   low 
  ridge 
  along 
  the 
  exterior 
  lateral 
  border, 
  also 
  along 
  the 
  articular 
  

   inner 
  border, 
  or 
  showing 
  a 
  more 
  or 
  less 
  wide 
  convexity 
  in 
  the 
  latter 
  

   region, 
  and 
  sometimes 
  presenting 
  a 
  more 
  or 
  less 
  well 
  defined 
  trans- 
  

   verse 
  prominence 
  in 
  mature 
  maxillary 
  form. 
  The 
  marginal 
  limits 
  

   of 
  the 
  crown 
  are 
  well 
  defined, 
  rounded 
  along 
  the 
  exterior 
  of 
  lateral 
  

   border 
  and 
  usually 
  inbeveled, 
  and 
  almost 
  always 
  making 
  an 
  angu- 
  

   lation 
  at 
  the 
  articular 
  inner 
  border 
  and 
  along 
  the 
  anterior 
  and 
  pos- 
  

   terior 
  margins, 
  the 
  enamel 
  extending 
  well 
  down 
  and 
  more 
  or 
  less 
  

   distinctly 
  defined 
  from 
  the 
  coarse 
  vernacularly 
  pitted 
  basis, 
  which 
  

   constitutes 
  the 
  greater 
  part 
  of 
  the 
  height 
  of 
  the 
  tooth. 
  In 
  front 
  and 
  

   behind, 
  the 
  basal 
  wall 
  is 
  nearly 
  exactly 
  vertical 
  to 
  the 
  plane 
  of 
  the 
  

   coronal 
  surface 
  and 
  moderately 
  channeled 
  or 
  concave 
  ; 
  the 
  inner 
  

   articular 
  face 
  is 
  also 
  vertical 
  and 
  slightly 
  excavated, 
  presenting 
  gen- 
  

   erally 
  at 
  one 
  or 
  the 
  other 
  extremity 
  an 
  obliquely 
  truncated 
  articular 
  

   facet 
  for 
  co-adaptation 
  with 
  the 
  contiguous- 
  tooth 
  of 
  the 
  opposite 
  

   series, 
  the 
  extent 
  and 
  obliquity 
  of 
  the 
  truncation 
  varying 
  greatly 
  

   according 
  to 
  the 
  species 
  ; 
  the 
  exterior 
  lateral 
  border, 
  in 
  typical 
  forms, 
  

   shows 
  an 
  expansion 
  of 
  the 
  basal 
  portion 
  beyond 
  the 
  coronal 
  limits, 
  

   increasing 
  in 
  breadth 
  and 
  terminating 
  in 
  a 
  more 
  or 
  less 
  produced 
  

   spur 
  at 
  the 
  postero-outer 
  angle 
  of 
  the 
  tooth. 
  The 
  coronal 
  surface 
  

   exhibits 
  under 
  an 
  ordinary 
  lens 
  a 
  distinct 
  vertical 
  prismatic 
  struc- 
  

   ture, 
  each 
  of 
  the 
  vertical 
  columns 
  enclosing 
  a 
  medullary 
  tube 
  the 
  

   appearance 
  of 
  which 
  at 
  the 
  surface 
  produces 
  the 
  exceedingly 
  minute 
  

   punctation 
  usually 
  observed 
  in 
  these 
  teeth 
  ; 
  the 
  exceedingly 
  elegant 
  

   vermiculose 
  rugosity 
  exhibited 
  in 
  the 
  less 
  worn 
  surfaces 
  of 
  certain 
  

  

  