﻿154 
  

  

  ON 
  AMMONITES 
  NAVICULARIS, 
  MANTELL. 
  

  

  By 
  the 
  late 
  G. 
  C. 
  Ckick, 
  F.G.S. 
  

  

  Bead 
  13th 
  Jime, 
  1919. 
  

  

  PLATE 
  IV. 
  

  

  1822. 
  Ammonites 
  navicuiaris, 
  G. 
  A. 
  Mantell, 
  Foss. 
  S. 
  Downs, 
  p. 
  198, 
  

  

  pi. 
  xxii, 
  fig. 
  5. 
  

   1827. 
  A. 
  tiavicularis, 
  J. 
  de 
  C. 
  Sowerby, 
  Min. 
  Conch., 
  vol. 
  tI, 
  p. 
  105, 
  

  

  pi. 
  DLV, 
  fig. 
  2. 
  

   1856. 
  A. 
  navicuiaris, 
  D. 
  Sharpe, 
  Foss. 
  Moll. 
  Chalk 
  of 
  England 
  

  

  (Mon. 
  Pal. 
  Soc), 
  pt. 
  iii, 
  1856, 
  p. 
  39, 
  pi. 
  xviii, 
  figs. 
  1-2. 
  

   1900. 
  Calycoceras 
  naviculare, 
  A. 
  Hyatt 
  in 
  Eastman's 
  translation 
  of 
  

   Zi'ttel's 
  " 
  Text-book 
  of 
  Palaiontology 
  ", 
  vol. 
  i, 
  p. 
  589, 
  and 
  

   Mon. 
  U.S. 
  Geol. 
  Surv., 
  vol. 
  xliv, 
  1903, 
  p. 
  113. 
  

   |-#^jf 
  The 
  following 
  specimens 
  in 
  the 
  National 
  Collection 
  seem 
  to 
  

   belong 
  to 
  this 
  species, 
  viz.: 
  5680, 
  Mantell 
  coll., 
  loc. 
  ? 
  ; 
  

   5681, 
  Mantell 
  coll.. 
  Upper 
  Chalk, 
  Offham 
  ; 
  30856-58, 
  Dixon 
  

   Coll., 
  Sussex.] 
  

  

  The 
  imperfection 
  of 
  Mantell's 
  type-specimen 
  of 
  Ammonites 
  

   navicuiaris, 
  its 
  assignment 
  by 
  its 
  author 
  to 
  the 
  Upper 
  Chalk, 
  and 
  the 
  

   poorness 
  of 
  the 
  figure 
  accompanying 
  the 
  author's 
  description, 
  have 
  

   given 
  rise 
  to 
  various 
  interpretations 
  of 
  the 
  species. 
  Thus 
  Pictet 
  

   regarded 
  it 
  as 
  an 
  Upper 
  Clialk 
  Ammonite 
  having 
  broad 
  whorls, 
  

   ornamented 
  with 
  very 
  coarse 
  rounded 
  ribs 
  wider 
  than 
  the 
  interspaces 
  

   between 
  them, 
  and 
  quite 
  distinct 
  from 
  the 
  specimens 
  which 
  Sowerby 
  

   and 
  Sliarpe 
  subsequentlj^ 
  figured 
  under 
  the 
  same 
  name. 
  

  

  Mantell's 
  description^ 
  of 
  the 
  species 
  is 
  as 
  follows: 
  — 
  

  

  "Elliptical? 
  umbilicate, 
  volutions 
  narrow, 
  compressed, 
  deeply 
  

   inserted, 
  rapidly 
  enlarging 
  ; 
  ambit 
  convex, 
  very 
  broad, 
  transversely 
  

   costated 
  ; 
  costte 
  numerous, 
  smooth, 
  rounded. 
  

  

  " 
  The 
  specimen 
  figured 
  is 
  tlie 
  only 
  known 
  example 
  of 
  this 
  species, 
  

   and 
  this 
  is 
  unfortunately 
  imperfect; 
  it 
  is, 
  however, 
  remarkably 
  

   characterized 
  by 
  its 
  navicular 
  form, 
  the 
  width 
  of 
  the 
  ambit, 
  large 
  

   rounded 
  costae, 
  and 
  sudden 
  increase 
  of 
  the 
  outer 
  volution. 
  The 
  ribs 
  

   are 
  perfectly 
  smooth, 
  and 
  so 
  numerous, 
  as 
  almost 
  to 
  expand 
  into 
  

   each 
  other; 
  with 
  but 
  few 
  exceptions, 
  they 
  extend 
  entirely 
  across 
  

   the 
  ambit, 
  forming 
  a 
  t'uberculated 
  margin 
  on 
  each 
  side 
  the 
  wreaths. 
  

   The 
  form 
  of 
  the 
  septa, 
  aperture, 
  and 
  umbilicus 
  is 
  unknown. 
  The 
  

   drawing 
  is 
  diminished 
  to 
  one-half 
  the 
  size 
  of 
  the 
  original. 
  

  

  "Zoca/%.— 
  Upper 
  Chalk, 
  Offham." 
  

  

  Mantell's 
  figure 
  of 
  the 
  type-specimen 
  is 
  very 
  misleading. 
  The 
  

   fossil, 
  which 
  is 
  now 
  in 
  the 
  British 
  Museum 
  collection 
  [register 
  

   number 
  5681], 
  is 
  very 
  much 
  depressed, 
  having 
  been 
  distorted 
  by 
  

   pressure; 
  its 
  longest 
  diameter 
  is 
  149mm.; 
  many 
  of 
  the 
  ribs 
  are 
  

   very 
  much 
  worn, 
  but 
  some 
  that 
  are 
  well-preserved 
  show 
  that 
  

  

  1 
  G. 
  A. 
  Mantell, 
  Foss. 
  S. 
  Downs, 
  1822, 
  p. 
  198, 
  pi. 
  xxii, 
  fig. 
  5. 
  Named 
  by 
  

   error 
  A. 
  catinus 
  in 
  the 
  explanation 
  of 
  the 
  plates. 
  

  

  