﻿SPONGIIDA. 
  479 
  

  

  a 
  new 
  genus 
  by 
  W. 
  Weltner 
  ('Beitr. 
  zur 
  Kenntniss 
  d, 
  Spongien,' 
  

   Inaugural 
  Dissertation, 
  Freiburg-in-Ereisgau, 
  8vo, 
  1882, 
  p. 
  30, 
  

   pi. 
  iii. 
  figs. 
  29-41, 
  43), 
  from 
  Prof. 
  Agassiz's 
  dredgings 
  in 
  1879 
  

   off 
  the 
  Morro 
  Light, 
  Gulf 
  of 
  Mexico, 
  in 
  250-500 
  fathoms. 
  In 
  

   point 
  of 
  fact 
  Weltner's 
  species, 
  which 
  differs 
  from 
  T. 
  dissimilis 
  prin- 
  

   cipally 
  in 
  the 
  possession 
  of 
  a 
  fully 
  developed 
  triradiate 
  "anchor," 
  

   occupies 
  a 
  position 
  almost 
  exactly 
  intermediate 
  between 
  T. 
  dissimilis 
  

   and 
  Stewart's 
  species. 
  I 
  gather 
  from 
  Weltner's 
  paper 
  that 
  he 
  has 
  

   not 
  seen 
  Stewart's 
  description 
  ; 
  had 
  he 
  done 
  so 
  he 
  would, 
  I 
  feel 
  

   sure, 
  have 
  at 
  any 
  rate 
  mentioned 
  the 
  close 
  affinity 
  of 
  his 
  species 
  

   to 
  that 
  of 
  Stewart, 
  from 
  which 
  it 
  differs 
  chiefly 
  by 
  the 
  elongate 
  

   form 
  of 
  the 
  flesh 
  -stellate 
  and 
  by 
  the 
  suppression 
  of 
  the 
  third 
  lateral 
  

   arm 
  of 
  the 
  skeleton-spicule, 
  a 
  suppression 
  already 
  foreshadowed 
  in 
  

   Stewart's 
  species 
  by 
  the 
  great 
  reduction 
  of 
  two 
  out 
  of 
  the 
  three 
  

   lateral 
  ai'ms 
  in 
  some 
  of 
  these 
  sjjiciiles 
  (see 
  fig. 
  75, 
  I. 
  c). 
  I 
  do 
  not 
  

   think 
  that 
  TribracJiium 
  can 
  be 
  upheld 
  as 
  distinct 
  from 
  Tetliyopsis 
  ; 
  

   the 
  gradation 
  of 
  forms 
  between 
  T. 
  columnifera 
  and 
  T. 
  dissimilis, 
  

   by 
  which 
  (1) 
  the 
  quadriradiate 
  spicule 
  of 
  T. 
  columnifera 
  is 
  reduced 
  

   to 
  a 
  triradiate 
  in 
  Tribrachium, 
  and 
  to 
  (a) 
  a 
  biradiate 
  with 
  aborted 
  

   third 
  ray 
  and 
  {b) 
  an 
  acerate 
  in 
  T. 
  dissimilis, 
  together 
  with 
  the 
  gene- 
  

   ral 
  agreement 
  between 
  the 
  minute 
  spicules, 
  the 
  skeletal 
  structure, 
  

   and 
  the 
  general 
  form 
  of 
  the 
  sponge, 
  appear 
  to 
  mark 
  these 
  three 
  

   species 
  out 
  as 
  belonging 
  to 
  a 
  natural 
  though 
  highly 
  plastic 
  circle 
  

   of 
  forms 
  com])arable 
  to 
  the 
  Tetractiuellid 
  genus 
  Placina, 
  Schulze, 
  

   of 
  which 
  the 
  species 
  (P. 
  inonolopha, 
  dilopha, 
  and 
  trilopJia, 
  Schulze) 
  

   each 
  include 
  hi-, 
  tri-, 
  and 
  quadriradiate 
  forms 
  of 
  the 
  fundamental 
  

   quadriradiate 
  type 
  ; 
  they 
  are 
  comparable 
  also 
  to 
  many 
  genera 
  of 
  

   the 
  Calcarea, 
  where 
  the 
  fundamental 
  (probably 
  triradiate) 
  type 
  ex- 
  

   hibits 
  great 
  modifications, 
  even 
  within 
  the 
  limits 
  of 
  a 
  single 
  species. 
  

   Besides 
  possessing 
  three 
  complete 
  arms 
  and 
  the 
  large 
  skeleton- 
  

   spicule, 
  Tribrachium 
  schmidti 
  is 
  distinguished 
  from 
  Tethi/opsis 
  dis- 
  

   similis 
  by 
  :— 
  (2) 
  the 
  exterior 
  being 
  unmarked 
  by 
  horizontal 
  ridges 
  ; 
  

  

  (3) 
  the 
  inferior 
  length 
  of 
  the 
  lateral 
  arm 
  of 
  the 
  triradiate 
  spicule 
  ; 
  

  

  (4) 
  the 
  apparent 
  absence 
  of 
  the 
  long 
  acerate 
  spicule 
  ; 
  (5) 
  the 
  more 
  

   generally 
  elongate 
  form 
  of 
  the 
  flesh-spicule 
  and 
  the 
  superior 
  number 
  

   of 
  its 
  lateral 
  whorls 
  of 
  tubercles. 
  

  

  Weltner's 
  comparison 
  of 
  the 
  form 
  of 
  the 
  minute 
  flesh-spicules 
  

   with 
  the 
  similarly 
  dendritic 
  skeleton-spicules 
  of 
  the 
  Ilhizomorine 
  

   Lithistids 
  is 
  invalidated 
  by 
  the 
  fact 
  that 
  the 
  two 
  classes 
  of 
  spicules 
  

   are 
  not 
  homologous 
  with 
  each 
  other, 
  the 
  flesh-spicules 
  o^Trihrachium 
  

   being 
  represented 
  in 
  the 
  Lithistid 
  series 
  only 
  by 
  the 
  minute 
  biha- 
  

   mates 
  and 
  other 
  flesh-spicules 
  of 
  Corallistes 
  &c. 
  

  

  A 
  striking 
  analogy 
  with 
  the 
  arrangement 
  of 
  the 
  skeleton 
  of 
  

   the 
  Lyssakiue 
  Hexactinellida 
  is 
  aftbrded 
  by 
  the 
  manner 
  in 
  which 
  

   the 
  arms 
  and 
  shafts 
  of 
  the 
  large 
  skeleton-spicules 
  are 
  employed 
  in 
  

   Tetliyopsis 
  (incl. 
  Tribrachium) 
  to 
  form 
  coherent 
  rectangular 
  meshes. 
  

  

  Weltner's 
  discovery 
  is 
  of 
  great 
  interest, 
  apart 
  from 
  the 
  ])eculiarities 
  

   of 
  the 
  type 
  described, 
  in 
  the 
  fact 
  that 
  his 
  species, 
  though 
  living 
  in 
  

   the 
  West 
  Indies, 
  is 
  clearly 
  intermediate 
  between 
  two 
  types 
  found 
  

   near 
  the 
  confines 
  of 
  the 
  Indo-Australian 
  region. 
  

  

  