﻿E. 
  Bose 
  — 
  Ammonoids. 
  55 
  

  

  group 
  of 
  G. 
  subcavum 
  Miller 
  and 
  Gurley 
  and 
  is 
  so 
  simi- 
  

   lar 
  to 
  the 
  type 
  that 
  at 
  first 
  I 
  thought 
  it 
  might 
  even 
  be 
  

   specifically 
  identical 
  with 
  it, 
  but 
  a 
  close 
  study 
  showed 
  a 
  

   number 
  of 
  small 
  but 
  constant 
  differences 
  which 
  allow 
  us 
  

   to 
  separate 
  it 
  under 
  the 
  name 
  of 
  Gastrioceras 
  subtilicos- 
  

   tatum 
  n. 
  sp. 
  

  

  The 
  group 
  of 
  G. 
  subcavum 
  is 
  characterized 
  by 
  ex- 
  

   tremely 
  angular 
  umbilical 
  shoulders 
  and 
  a 
  wide, 
  deep, 
  

   funnel-shaped 
  umbilicus 
  without 
  strong 
  nodules 
  on 
  its 
  

   border. 
  Our 
  species 
  shows 
  both 
  these 
  characteristics, 
  

   but 
  the 
  umbilicus 
  is 
  still 
  a 
  little 
  wider 
  than 
  in 
  the 
  type. 
  

   The 
  cross-section 
  is 
  nearly 
  the 
  same, 
  although 
  the 
  inner 
  

   whorls 
  have 
  a 
  venter 
  that 
  is 
  possibly 
  a 
  little 
  more 
  

   rounded. 
  The 
  suture 
  is 
  very 
  similar 
  in 
  all 
  species 
  of 
  

   Gastrioceras 
  but 
  the 
  siphonal 
  saddle 
  is 
  larger 
  than 
  that 
  

   figured 
  by 
  Smith, 
  5 
  and 
  as 
  the 
  author 
  says 
  that 
  G. 
  sub- 
  

   cavum 
  agrees 
  with 
  G. 
  globulosum 
  Miller 
  and 
  Gurley 
  in 
  

   its 
  septa, 
  I 
  presume 
  that 
  his 
  figure 
  is 
  taken 
  from 
  an 
  im- 
  

   mature 
  specimen. 
  The 
  suture 
  of 
  our 
  species 
  is 
  very 
  sim- 
  

   ilar 
  to 
  that 
  of 
  G. 
  globulosum 
  except 
  that 
  the 
  siphonal 
  

   saddle 
  is 
  somewhat 
  stouter. 
  

  

  The 
  main 
  difference 
  between 
  the 
  New 
  Mexico 
  species 
  

   and 
  G. 
  subcavum 
  appears 
  to 
  consist 
  in 
  the 
  ornamentation 
  

   and 
  the 
  form 
  of 
  the 
  constrictions. 
  Our 
  species 
  shows 
  a 
  

   greater 
  number 
  of 
  constrictions 
  (5 
  to 
  7) 
  and 
  these 
  are 
  

   strongly 
  curved 
  forward 
  on 
  the 
  venter, 
  while 
  in 
  G. 
  sub- 
  

   cavum 
  there 
  are 
  only 
  about 
  four 
  constrictions, 
  which 
  

   show 
  a 
  very 
  slight 
  sinuosity. 
  

  

  The 
  differences 
  in 
  ornamentation 
  are 
  still 
  greater. 
  G. 
  

   subcavum 
  shows 
  only 
  very 
  faint 
  lines 
  of 
  growth 
  and 
  ac- 
  

   cording 
  to 
  Smith 
  has 
  no 
  umbilical 
  ribs. 
  Our 
  species, 
  on 
  

   the 
  contrary, 
  shows 
  a 
  very 
  distinct 
  ornamentation. 
  On 
  

   the 
  umbilical 
  wall 
  we 
  count 
  over 
  thirty 
  very 
  fine 
  but 
  well 
  

   defined 
  radial 
  ribs 
  or 
  costse 
  which 
  on 
  the 
  umbilical 
  

   shoulder 
  swell 
  into 
  small 
  nodules; 
  from 
  each 
  of 
  these, 
  

   one 
  or 
  two 
  even 
  bundles 
  of 
  three 
  and 
  four 
  fine 
  costse 
  

   (much 
  finer 
  still 
  than 
  those 
  on 
  the 
  umbilical 
  wall) 
  start 
  

   and 
  run 
  over 
  the 
  venter 
  strongly 
  curved 
  forward. 
  All 
  

   this 
  ornamentation 
  is 
  very 
  coarse 
  in 
  immature 
  specimens 
  

   but 
  in 
  adults 
  it 
  becomes 
  gradually 
  finer 
  and 
  on 
  fragments 
  

   of 
  the 
  largest 
  individuals 
  where 
  the 
  shell 
  is 
  well 
  pre- 
  

   served 
  the 
  fine 
  umbilical 
  ribs 
  are 
  rather 
  distant 
  from 
  

   each 
  other 
  but 
  very 
  distinct, 
  although 
  extremely 
  delicate. 
  

  

  5 
  Op. 
  cit., 
  p. 
  97, 
  pi. 
  17, 
  figs. 
  15-17. 
  

  

  