﻿GROWTH 
  AND 
  MIGEATION 
  OF 
  FRESH-WATER 
  MUSSELS. 
  

  

  17 
  

  

  ARRESTED 
  GROWTH 
  RINGS. 
  

  

  "Growth 
  lines," 
  the 
  conspicuous 
  dark 
  concentric 
  rings 
  of 
  the 
  shell, 
  

   may 
  be 
  due 
  to 
  (1) 
  thick 
  epidermis, 
  (2) 
  double 
  epidermis, 
  and 
  espe- 
  

   cially 
  (3) 
  to 
  double 
  epidermal 
  and 
  prismatic 
  layers. 
  It 
  is 
  well 
  

   known 
  that 
  the 
  epidermal 
  and 
  prismatic 
  layers 
  are 
  formed 
  by 
  the 
  

  

  .,.--^.. 
  

  

  --9---. 
  

  

  Fig. 
  1-4. 
  — 
  Cross-sections 
  of 
  shells 
  of 
  Quadrula 
  showing 
  structure 
  inregion 
  of 
  rest 
  rings, 
  all 
  figures 
  enlarged 
  

   20 
  times; 
  e, 
  epidermis; 
  p, 
  prismaticlayer; 
  n, 
  nacreous 
  layers; 
  m', 
  successive 
  layers 
  of 
  nacreous 
  structure' 
  

   g, 
  position 
  and 
  width 
  of 
  rest 
  rings 
  (growth 
  lines). 
  

  

  Fig. 
  1. 
  — 
  Section 
  from 
  near 
  edge 
  of 
  shell, 
  showing 
  double 
  layers 
  and 
  long 
  underlying 
  tongue 
  of 
  epidermal 
  

   and 
  prismatic 
  structure, 
  which 
  formed 
  the 
  dark 
  wide 
  rest 
  ring 
  of 
  a 
  young 
  rapid-growing 
  Quadrula 
  lachry- 
  

   mosa. 
  Rings 
  1 
  1 
  or 
  1=, 
  specimens 
  149 
  or 
  ICS, 
  plate 
  n, 
  would 
  give 
  sunOar 
  sections. 
  

  

  Fig. 
  2. 
  — 
  Section 
  of 
  shell 
  of 
  mature 
  mussel, 
  taken 
  about 
  the 
  middle 
  of 
  the 
  valve. 
  Rest 
  ring 
  not 
  so 
  wide 
  

   as 
  in 
  1, 
  tongue 
  shorter, 
  prismatic 
  layer 
  thicker, 
  and 
  thick 
  successive 
  layers 
  of 
  nacreous 
  struc.ure. 
  

  

  Fig. 
  3. 
  — 
  A 
  rest 
  ring 
  due 
  chiefly 
  to 
  unusually 
  thick, 
  double 
  layers 
  of 
  epidermis. 
  

  

  Fig. 
  4. 
  — 
  Two 
  successive 
  rest 
  rings 
  near 
  together, 
  undoubtedly 
  formed 
  in 
  the 
  same 
  season, 
  and 
  probably 
  

   only 
  a 
  few 
  weeks 
  apart, 
  as 
  specimen 
  was 
  young 
  and 
  at 
  the 
  rapid-growing 
  age. 
  (See 
  description 
  of 
  specimen 
  

   413, 
  pi. 
  n.) 
  

  

  edge 
  of 
  the 
  mantle 
  only. 
  Thus 
  increase 
  in 
  shell 
  diameter 
  begins 
  

   with 
  the 
  formation 
  of 
  the 
  epidermal, 
  followed 
  by 
  the 
  prismatic 
  layer. 
  

   However, 
  if 
  the 
  mantle 
  is 
  withdrawn 
  from 
  the 
  edge 
  of 
  the 
  shell, 
  it 
  

   often 
  puts 
  down 
  new 
  layers 
  of 
  epidermal 
  and 
  prismatic 
  material 
  

  

  