﻿410 
  PROCEEDINGS 
  OF 
  THE 
  GEOLOGICAL 
  SOCIETY. 
  [June 
  8, 
  

  

  therefore, 
  than 
  the 
  Microlestian 
  teeth 
  of 
  Frome 
  and 
  Diegerloch, 
  it 
  is 
  

   the 
  earliest-known 
  trace 
  of 
  a 
  fossil 
  Mammal 
  in 
  the 
  Secondary 
  rocks. 
  

  

  § 
  III. 
  Description. 
  

  

  The 
  crown 
  (see 
  fig. 
  3), 
  oblong 
  in 
  shape, 
  is 
  very 
  long 
  in 
  pro- 
  

   portion 
  to 
  its 
  width, 
  being 
  1*5 
  inch 
  long 
  to 
  04 
  wide. 
  The 
  higher 
  

   side 
  of 
  its 
  obliquely 
  worn 
  summit, 
  imbedded 
  in, 
  or 
  rather 
  adherent 
  

   to, 
  the 
  stone, 
  exhibits 
  two 
  isolated 
  involutions 
  of 
  enamel 
  on 
  that 
  

   portion 
  of 
  the 
  tooth 
  that 
  is 
  supported 
  by 
  the 
  posterior 
  fang. 
  Ante- 
  

   rior 
  to 
  these 
  are 
  two 
  wider 
  and 
  less 
  prominent 
  folds. 
  The 
  anterior 
  

   corner, 
  unfortunately 
  broken 
  by 
  the 
  waves, 
  may 
  perhaps 
  have 
  borne 
  

   an 
  additional 
  fold. 
  The 
  cervix 
  is 
  very 
  well 
  defined. 
  Of 
  the 
  two 
  

   divergent 
  fangs, 
  the 
  anterior 
  had 
  been 
  broken 
  short 
  off 
  before 
  it 
  was 
  

   imbedded 
  in 
  the 
  matrix 
  ; 
  the 
  posterior 
  or 
  smaller 
  of 
  the 
  two 
  (in 
  

   length 
  0*11 
  inch) 
  is 
  perfect, 
  and 
  haS" 
  its 
  tip 
  slightly 
  reflected. 
  

   There 
  is 
  no 
  evidence 
  as 
  to 
  its 
  position 
  in 
  the 
  jaw. 
  

  

  § 
  IV. 
  Determination. 
  

  

  The 
  former 
  discovery 
  of 
  Marsupials 
  in 
  the 
  Secondary 
  rocks 
  natu- 
  

   rally 
  inclined 
  me 
  to 
  seek 
  the 
  existing 
  analogue 
  of 
  this 
  bone 
  and 
  

   mutilated 
  stump 
  in 
  that 
  great 
  order, 
  now, 
  with 
  the 
  exception 
  of 
  the 
  

   Opossum, 
  confined 
  to 
  the 
  southern 
  hemisphere 
  ; 
  and 
  this 
  inference 
  

   has 
  been 
  verified 
  by 
  a 
  rigid 
  comparison. 
  The 
  small 
  well-marked 
  

   folds 
  on 
  the 
  higher 
  side 
  of 
  the 
  tooth, 
  coupled 
  with 
  its 
  oblique 
  grind- 
  

   ing 
  surface 
  and 
  its 
  great 
  length 
  and 
  narrowness, 
  point, 
  and 
  point 
  

   only, 
  towards 
  the 
  Macropoda 
  of 
  Van 
  der 
  Hoeven 
  (the 
  Kangaroos) 
  and 
  

   the 
  Hypsiprymnidce, 
  or 
  Kangaroo 
  -rats. 
  Of 
  the 
  former 
  genus, 
  the 
  

   first 
  trenchant 
  bifanged 
  milk-molar 
  approximates 
  closely 
  to 
  the 
  fossil 
  

   in 
  the 
  waved 
  outline 
  which 
  it 
  would 
  present 
  if 
  the 
  inner 
  and 
  posterior 
  

   extension 
  of 
  the 
  trenchant 
  edge 
  were 
  removed 
  by 
  wear 
  ; 
  but 
  the 
  

   section, 
  in 
  that 
  case, 
  would 
  be 
  trihedral, 
  instead 
  of 
  being 
  oblong 
  as 
  

   in 
  the 
  fossil. 
  The 
  absence 
  of 
  plication 
  on 
  the 
  permanent 
  premolar 
  

   of 
  Macropus, 
  apart 
  from 
  its 
  great 
  transverse 
  extent, 
  puts 
  that 
  genus 
  

   out 
  of 
  the 
  comparison. 
  The 
  trenchant 
  plicated 
  premolar, 
  on 
  the 
  

   other 
  hand, 
  of 
  Hypsiprymnus 
  is 
  impressed 
  with 
  all 
  the 
  main 
  charac- 
  

   teristics 
  of 
  the 
  fossil. 
  The 
  oblique 
  wear 
  of 
  the 
  crown, 
  the 
  great 
  

   length 
  as 
  compared 
  with 
  the 
  breadth, 
  the 
  plicae, 
  and 
  the 
  implanta- 
  

   tion 
  by 
  two 
  fangs 
  are 
  seen 
  alike 
  in 
  both 
  (see 
  figs. 
  3-5). 
  But, 
  

   while 
  the 
  general 
  correspondence 
  in 
  form 
  is 
  so 
  marked, 
  the 
  minor 
  

   differences 
  are 
  by 
  no 
  means 
  unimportant. 
  One 
  section 
  of 
  the 
  

   Hypsiprymnidce 
  [jET. 
  Gaimardi, 
  H. 
  Hunteri, 
  Bettongia 
  Grayi, 
  B. 
  

   penicillata, 
  &c] 
  presents 
  seven 
  narrow 
  plicae, 
  a 
  second 
  [H. 
  minor 
  

   (=H. 
  murinus, 
  Owen), 
  Bettongia 
  rufescens, 
  &c] 
  four 
  wide 
  plicae 
  

   respectively 
  on 
  their 
  premolars. 
  In 
  the 
  small 
  number 
  of 
  folds 
  (four, 
  

   or 
  perhaps 
  five), 
  and 
  in 
  their 
  width, 
  the 
  fossil 
  points 
  away 
  from 
  the 
  

   former 
  towards 
  the 
  latter 
  of 
  these 
  ; 
  it 
  differs 
  from 
  the 
  latter 
  in 
  the 
  

   fact 
  that 
  two 
  of 
  the 
  plicae 
  are 
  supported 
  by 
  the 
  posterior 
  fang, 
  while 
  

   in 
  the 
  recent 
  four-plicated 
  premolars 
  all 
  the 
  plicae 
  are 
  supported 
  by 
  

   the 
  stout 
  anterior 
  fang. 
  Figs. 
  4 
  a 
  and 
  4 
  6* 
  represent 
  the 
  unworn 
  

   * 
  Nos. 
  1783-84 
  of 
  Hunterian 
  Catalogue. 
  

  

  