28  2 
MR.  R.  LYDEKKER  ON  A  MAMMALIAN 
[Aug.  1893, 
inner  aspect  of  the  tooth.  The  summit  of  the  tooth  appears  to  have 
the  oblique  wear  characterizing  the  incisors  of  Eodents,  while  the 
lower  and  larger  extremity  seems  to  have  been  open.  We  have 
here,  in  fact,  an  incisor  which  evidently  grew  from  a  persistent 
pulp,  and  which  can  in  no  way  be  distinguished  from  that  of  a 
Rodent.  That  the  tooth  is  mammalian  there  cannot  be  the  slightest 
doubt,  as  no  reptile  exhibits  any  approximation  to  this  type  of 
dental  structure ;  and,  so  far  as  I  can  see,  there  is  no  possibility  of 
distinguishing  it  from  a  Rodent  incisor.  In  this  opinion,  I  am 
permitted  to  state,  I  have  the  support  of  my  friends,  Dr.  Forsyth 
Major  and  Mr.  Oldfield  Thomas. 
As  it  would,  however,  be  in  the  highest  degree  improbable  to 
meet  with  Rodents  in  the  Wealden,  we  naturally  turn  to  the 
Mesozoic  Multituberculata  as  being  the  group  to  which  the  specimen 
is  most  likely  to  belong,  seeing  that  these  mammals  were  furnished 
with  more  or  less  scalpriform  incisors. 
Now,  in  Plcigiciulax  and  the  allied  or  identical  Ctenacodon  of 
North  America,  the  lower  incisors  (which  are  the  only  ones  at 
present  known)  are  quite  unlike  the  specimen  under  consideration, 
and  it  may  be  inferred  that  the  corresponding  upper  teeth  would 
likewise  have  been  of  a  different  type.  I  find,  however,  that  from 
the  Upper  Jurassic  of  North  America  Prof.  Marsh  1  has  described 
and  figured  a  Rodent-like  incisor  tooth,  which  he  refers  with  the 
greatest  probability  of  correctness  to  the  genus  Allodon , — a  genus,  by 
the  way,  which  I  fail  to  distinguish  from  Bolodon  of  the  English 
Purbeck.  In  his  description  of  the  specimen  in  question,  which  is 
here  represented  in  figs.  2,  2  a,  p.  281,  Prof.  Marsh  writes  that  it 
“  is  faced  with  enamel  in  front,  and  grew  from  a  persistent  pulp, 
like  the  incisor  of  a  Rodent.  The  summit  is  incomplete,  and  hence 
the  shape  of  the  worn  surface  cannot  be  determined.”  In  the 
figures  of  the  American  tooth  the  enamel  is  represented  as  stopping 
short  of  the  lower  extremity;  but  this  is  doubtless  due  to  the 
imperfection  of  the  particular  specimen.  Otherwise,  the  two  specimens 
accord  both  in  form  and  in  size  ;  and  there  can  be  no  doubt  that  they 
belong  to  closely  allied  animals.  In  the  upper  jaw  of  the  so-called 
Allodon  there  are  three  pairs  of  incisors,  of  which  the  middle  one 
is  larger  than  either  of  the  others.  It  is  against  this  large  middle 
upper  incisor  that  the  lower  tooth  of  the  type  under  consideration 
is  presumed  to  have  bitten.  As  there  is  no  other  known  genus  of 
mammal  to  which  this  type  of  tooth  could  have  belonged,  I  am  dis¬ 
posed  to  endorse  Prof.  Marsh’s  reference  of  the  American  specimen 
to  Allodon ;  and  I  accordingly  assign  the  English  example  to 
Bolodon.  Unfortunately,  we  cannot  compare  the  Wealden  Bolodon 
with  B.  crassidens  of  the  Purbeck,  which  is  described  on  the  evi¬ 
dence  of  upper  jaws  ;  and  it  is  consequently  impossible  to  say 
whether  the  two  are  specifically  different.  If,  however,  this  should 
eventually  prove  to  be  the  case,  I  would  suggest  that  the  Wealden 
form  should  be  named  after  the  finder  of  the  present  specimen. 
1  Am.  Journ.  Sci.  ser.  3,  vol.  xxxiii.  (1887)  p.  331,  pi.  vii.  figs.  14,  15. 
