﻿812 
  Forty-seventh 
  Report 
  on 
  the 
  State 
  Museum. 
  

  

  either 
  or 
  both 
  valves, 
  and 
  may 
  be 
  supported 
  or 
  not 
  supported 
  

   by 
  a 
  median 
  septum. 
  

  

  The 
  deltidium 
  and 
  deltidial 
  plates, 
  though 
  similar 
  in 
  functions, 
  

   are 
  profoundly 
  distinct, 
  both 
  in 
  origin 
  and 
  structure. 
  The 
  former 
  

   is 
  primitive 
  and 
  fundamental, 
  the 
  latter 
  is 
  wholly 
  secondary 
  ; 
  a 
  

   replacement 
  of, 
  but 
  never 
  a 
  derivative 
  from 
  the 
  former. 
  In 
  the 
  

   foregoing 
  discussions 
  of 
  the 
  genera 
  these 
  parts 
  have 
  been 
  distin- 
  

   guished 
  simply 
  by 
  the 
  designations 
  generally 
  current; 
  the 
  term 
  

   deltidium 
  referring 
  exclusively 
  to 
  the 
  convex 
  external 
  portion 
  of 
  

   the 
  pedicle-sheath, 
  such 
  as 
  is 
  found 
  in 
  Clitambonites, 
  Siropho- 
  

   mena, 
  Rafinesquina, 
  and 
  their 
  allies, 
  and 
  which, 
  under 
  no 
  

   condition, 
  shows 
  evidence 
  of 
  composition 
  or 
  consolidation 
  of 
  

   separate 
  parts. 
  The 
  term 
  deltidial 
  plates 
  has 
  been 
  applied 
  to 
  

   that 
  condition 
  of 
  the 
  external 
  sheath 
  in 
  which 
  a 
  division 
  into 
  

   component 
  parts 
  is 
  evident, 
  as 
  in 
  Athyris, 
  Atrypa, 
  Merista, 
  

   the 
  terebratuloids, 
  etc. 
  ; 
  or 
  inferential, 
  as 
  in 
  Cyrtia 
  and 
  Cybtina. 
  

   The 
  terminology 
  is 
  here 
  so 
  imperfect 
  as 
  easily 
  to 
  cause 
  confusion, 
  

   and 
  though 
  it 
  had 
  not 
  seemed 
  needful 
  heretofore 
  to 
  suggest 
  an 
  

   improvement, 
  it 
  has 
  become 
  necessary, 
  for 
  the 
  proper 
  considera- 
  

   tion 
  of 
  the 
  subject, 
  to 
  employ 
  a 
  more 
  distinctive 
  expression 
  for 
  

   these 
  fundamentally 
  different 
  structures. 
  The 
  secondary 
  struc- 
  

   tures 
  known 
  as 
  the 
  deltidial 
  plates, 
  whether 
  already 
  discrete 
  as 
  

   in 
  the 
  terebratuloids, 
  rhynchonelloids 
  and 
  meristoids, 
  or 
  solidly 
  

   coalesced, 
  as 
  in 
  Nucleospira, 
  Parazyga, 
  Cyriia 
  and 
  Cyrtina, 
  

   will 
  henceforward 
  be 
  termed 
  the 
  deltarium, 
  in 
  application 
  to 
  the 
  

   parts 
  as 
  a 
  whole, 
  or 
  the 
  deltaria 
  in 
  referring 
  to 
  the 
  component 
  

   plates. 
  It 
  may 
  also 
  prove 
  convenient 
  to 
  adopt 
  the 
  term 
  

   introduced 
  by 
  Bronn, 
  pseudodeltidium, 
  for 
  the 
  coalesced 
  condi- 
  

   tion 
  of 
  the 
  deltaria 
  in 
  Spirifer, 
  Cyrtia, 
  etc., 
  as 
  this 
  is 
  its 
  original 
  

   meaning 
  ; 
  but 
  the 
  significance 
  of 
  the 
  term 
  will 
  be 
  subordinate 
  to 
  

   that 
  of 
  deltarium. 
  

  

  The 
  researches 
  by 
  Kowalevski* 
  upon 
  the 
  development 
  and 
  

   detailed 
  anatomy 
  of 
  Thecidea 
  (Lacazella) 
  and 
  Cistella 
  

   (=Argiope, 
  Kow.) 
  have 
  recently 
  been 
  interpreted 
  in 
  the 
  bearing 
  

   upon 
  these 
  structures 
  by 
  Beech 
  er, 
  who 
  has 
  also 
  added 
  new 
  data 
  

   derived 
  from 
  the 
  study 
  of 
  Magellania 
  flavescens 
  and 
  Terebratula 
  

   septentrionalis. 
  Thecidea, 
  or 
  Lacazella 
  Mediterranea, 
  is 
  the 
  

  

  * 
  Observations 
  on 
  the 
  Development 
  of 
  the 
  Brachiopoda; 
  Proceedings 
  of 
  the 
  Session 
  of 
  the 
  

   Imperial 
  Society 
  of 
  Amateur 
  Naturalists, 
  etc., 
  held 
  at 
  the 
  University 
  of 
  Moscow, 
  Eleventh 
  

   year, 
  vol. 
  XIV. 
  

  

  