﻿SONNINIA 
  AND 
  HAMMATOCERAS. 
  659 
  

  

  almost 
  obsolete 
  but 
  bordered 
  by 
  two 
  rudimentary 
  sulci. 
  Inner 
  mar- 
  

   gin 
  not 
  defined. 
  Inclusion 
  barely 
  one 
  fourth, 
  leaving 
  a 
  space 
  between 
  

   the 
  spines 
  and 
  the 
  succeeding 
  whorl. 
  

  

  The 
  above 
  description 
  applies 
  more 
  particularly 
  to 
  immature 
  

   specimens. 
  When 
  full-grown 
  the 
  whorls 
  become 
  subquadrate, 
  the 
  

   ventral 
  area 
  is 
  flatter 
  and 
  better 
  defined, 
  the 
  inner 
  margin 
  is 
  flat, 
  

   and 
  the 
  lateral 
  spines 
  give 
  place 
  to 
  very 
  coarse 
  ribs. 
  

  

  The 
  size 
  and 
  persistence 
  of 
  spines 
  which 
  are 
  not 
  in 
  contact 
  with 
  

   the 
  succeeding 
  whorl, 
  and 
  the 
  inner 
  margin 
  not 
  defined 
  until 
  late 
  in 
  

   life, 
  are 
  the 
  characteristics 
  of 
  this 
  species. 
  The 
  strong 
  spines 
  suffi- 
  

   ciently 
  separate 
  it 
  from 
  Sonn. 
  polyacantlia 
  ("Waagen), 
  which 
  is 
  

   probably 
  its 
  direct 
  descendant. 
  

  

  It 
  occurs 
  in 
  the 
  Concavum-zone 
  at 
  Bradford 
  Abbas, 
  Dorset, 
  and 
  

   reaches 
  a 
  diameter 
  of 
  nine 
  inches. 
  

  

  SONNINIA 
  SCTBTRIGOJSTATA, 
  n. 
  Sp. 
  

  

  Discoidal, 
  somewhat 
  compressed, 
  hollow-carinate. 
  Whorls 
  at 
  two 
  

   inches 
  diameter 
  almost 
  circular, 
  ornamented 
  with 
  arcuate 
  ribs, 
  which 
  

   are 
  directed 
  somewhat 
  backwards, 
  and 
  then 
  have 
  a 
  pronounced 
  

   forward 
  sweep 
  on 
  the 
  ventral 
  area 
  ; 
  on 
  the 
  middle 
  of 
  the 
  lateral 
  area 
  

   are 
  coarse, 
  irregular 
  spines. 
  After 
  a 
  diameter 
  of 
  two 
  inches, 
  the 
  

   whorls 
  are 
  more 
  triangular 
  in 
  shape 
  and 
  drawn 
  out 
  ventrally, 
  the 
  

   ribs 
  are 
  more 
  prominent, 
  and 
  large 
  irregular 
  bosses 
  appear 
  on 
  the 
  

   edge 
  of 
  the 
  inner 
  margin 
  ; 
  from 
  these 
  bosses 
  or 
  from 
  between 
  them 
  the 
  

   ribs 
  spring. 
  Ventral 
  area 
  not 
  defined. 
  Carina 
  hollow 
  and 
  promi- 
  

   nent, 
  on 
  the 
  core 
  nearly 
  obsolete, 
  and, 
  up 
  to 
  a 
  diameter 
  of 
  two 
  inches, 
  

   bordered 
  by 
  rudimentary 
  furrows. 
  Inner 
  margin 
  not 
  actually 
  

   defined. 
  Inclusion 
  about 
  two 
  fifths 
  — 
  the 
  spines 
  being 
  touched 
  by 
  

   the 
  succeeding 
  whorl. 
  

  

  This 
  species 
  is 
  somewhat 
  like 
  Quenstedt's 
  Amm. 
  Soiverbyi 
  trigon- 
  

   atus 
  (Schwabischen 
  Jura, 
  pi. 
  lxi. 
  f. 
  14) 
  ; 
  but 
  it 
  has 
  more 
  spines 
  on 
  

   the 
  inner 
  whorls, 
  is 
  more 
  coarsely 
  ornamented, 
  has 
  its 
  ribs 
  directed 
  

   more 
  backwards, 
  has 
  a 
  less 
  triangular 
  aperture 
  and 
  a 
  much 
  larger 
  

   carina. 
  The 
  greater 
  thickness, 
  coarser 
  ribs, 
  bosses 
  on 
  inner 
  area 
  

   instead 
  of 
  median 
  spines, 
  and 
  more 
  triangular 
  shape 
  separate 
  it 
  from 
  

   Sonn. 
  Soiverbyi. 
  

  

  The 
  inner 
  whorls 
  of 
  this 
  species 
  are 
  practically 
  a 
  somewhat 
  in- 
  

   cluded 
  Sonn. 
  acanthodes 
  in 
  miniature. 
  

  

  The 
  specimen 
  is 
  from 
  Sherborne, 
  probably 
  from 
  the 
  Concavum- 
  

   zone. 
  

  

  Part 
  II. 
  — 
  Hammatoceras. 
  

  

  The 
  descent 
  of 
  Hammatoceras 
  involves 
  the 
  supposition 
  of 
  an 
  actual 
  

   reversion 
  in 
  the 
  matter 
  of 
  ornament 
  and 
  in 
  the 
  size 
  of 
  the 
  umbilicus. 
  

   That 
  Hamm. 
  insigne 
  is 
  descended 
  from 
  Deroceras 
  armatum 
  seems 
  

   to 
  be 
  very 
  probable, 
  not 
  only 
  on 
  account 
  of 
  the 
  similarity 
  of 
  

   suture-line, 
  but 
  also 
  because 
  of 
  the 
  similarity 
  of 
  ornament. 
  Quen- 
  

   stedt 
  allows 
  us 
  to 
  form 
  some 
  idea 
  of 
  the 
  manner 
  in 
  which 
  Dero- 
  

  

  