﻿12 
  PROCEEDINGS 
  OF 
  THE 
  NATIONAL 
  MUSEUM 
  vol.98 
  

  

  length 
  or 
  sometimes 
  only 
  a 
  few 
  scales 
  anteriorly 
  (in 
  

   poorly 
  preserved 
  specimens 
  scaly 
  anal 
  sheath 
  may 
  

   be 
  lacking) 
  ; 
  four 
  glanrllike 
  depressions 
  on 
  top 
  of 
  

   snout, 
  sometimes 
  indistinct 
  in 
  poorly 
  preserved 
  

   specimens 
  and 
  represented 
  by 
  pores 
  in 
  2\ 
  colom- 
  

   Mensis; 
  scales 
  with 
  posterior 
  margins 
  dentate, 
  

   weakly 
  erenate 
  (except 
  in 
  the 
  young 
  the 
  scale 
  mar- 
  

   gins 
  entire, 
  or 
  in 
  adults 
  of 
  certain 
  species 
  margins 
  

   entire 
  except 
  for 
  a 
  row 
  or 
  so 
  of 
  crenulate 
  scales 
  

   along 
  middorsal 
  line) 
  ; 
  teeth 
  in 
  outer 
  row 
  a 
  little 
  

   enlarged, 
  then 
  a 
  narrow 
  band 
  of 
  villiform 
  teeth 
  in- 
  

   side 
  ; 
  vertebrae 
  in 
  several 
  counts 
  18 
  to 
  20 
  -f 
  19 
  to 
  22 
  

   (marine, 
  west 
  coast 
  of 
  Mexico 
  to 
  Panama 
  Bay 
  and 
  

   in 
  fresh 
  waters 
  of 
  Mexico 
  to 
  Central 
  America 
  ; 
  also 
  

   Atlantic 
  side 
  of 
  Guatemala 
  to 
  Tanama 
  in 
  streams). 
  

  

  Thyrinops 
  Hubbs 
  

   30l>. 
  Ascending 
  premaxillary 
  process 
  short, 
  broad-based, 
  

   and 
  triangular 
  in 
  shape; 
  lower 
  jaw 
  equal 
  to 
  or 
  a 
  

   trille 
  longer 
  than 
  upper 
  jaw, 
  not 
  included 
  ; 
  belly 
  

   rounded 
  ; 
  no 
  sheath 
  of 
  scales 
  along 
  anal-fin 
  base 
  ; 
  

   no 
  glandlike 
  structures 
  on 
  snout 
  as 
  in 
  30o 
  ; 
  margins 
  

   of 
  scales 
  entire; 
  teeth 
  in 
  outer 
  row 
  somewhat 
  en- 
  

   larged, 
  conical, 
  followed 
  by 
  a 
  widely 
  spaced 
  inner 
  

   row 
  on 
  lower 
  jaw, 
  but 
  on 
  upper 
  jaw 
  inner 
  teeth 
  

   irregular 
  or 
  in 
  a 
  narrow 
  villiform 
  band 
  ; 
  vertebrae 
  

   in 
  two 
  counts 
  15 
  or 
  16 
  -f 
  22 
  or 
  23 
  (Rio 
  Balsas, 
  Guer- 
  

   rero, 
  west 
  coast 
  of 
  Mexico) 
  Melaniris 
  Meek 
  

  

  28&. 
  Origin 
  of 
  first 
  dorsal 
  notably 
  in 
  front 
  of 
  anal 
  origin 
  to 
  over 
  

   base 
  of 
  first 
  branched 
  ray 
  of 
  anal 
  fin 
  ; 
  belly 
  rounded. 
  

   31a. 
  Lower 
  jaw 
  usually 
  angular, 
  somewhat 
  pointed 
  at 
  tip, 
  

   longer 
  than 
  upper, 
  notably 
  projecting 
  in 
  front 
  of 
  tip 
  of 
  

   snout 
  when 
  mouth 
  is 
  closed 
  ; 
  front 
  tip 
  of 
  premaxillaries 
  

   angular, 
  more 
  or 
  less 
  pointed 
  ; 
  body 
  cavity 
  extending 
  

   notably 
  past 
  anal 
  origin, 
  at 
  least 
  beyond 
  base 
  of 
  third 
  

   branched 
  anal 
  ray 
  ; 
  margins 
  of 
  scales 
  entire 
  or 
  crenu- 
  

   late 
  ; 
  no 
  scaly 
  sheath 
  along 
  base 
  of 
  anal 
  fin 
  ; 
  ascend- 
  

   ing 
  bony 
  process 
  of 
  premaxillaries 
  usually 
  narrow- 
  

   based 
  and 
  elongate 
  or 
  spinelike, 
  with 
  a 
  V-shaped 
  space 
  

   between 
  them 
  ; 
  teeth 
  in 
  both 
  jaws 
  in 
  two 
  or 
  three 
  ir- 
  

   regular 
  rows, 
  conical, 
  sometimes 
  enlarged; 
  vertebrae 
  

   in 
  several 
  counts 
  19 
  to 
  23 
  + 
  18 
  to 
  24 
  (fresh 
  waters 
  

   of 
  Rio 
  Lerma 
  Basin 
  and 
  Valley 
  of 
  Mexico). 
  

  

  Chirostoma 
  Swaiuson 
  

   31&. 
  Lower 
  jaw 
  a 
  little 
  longer 
  than 
  upper 
  or 
  a 
  little 
  shorter 
  or 
  

   included, 
  not 
  angular 
  but 
  with 
  rounded 
  tip 
  ; 
  front 
  tips 
  

   of 
  premaxillaries 
  rounded 
  ; 
  body 
  cavity 
  extending 
  to 
  

   opposite 
  anal-fin 
  origin 
  or 
  to 
  base 
  of 
  fifth 
  branched 
  

   anal 
  ray 
  in 
  Menidia 
  extensa; 
  margins 
  of 
  scales 
  entire 
  

   or 
  very 
  weakly 
  crenulate. 
  

   32a. 
  Lower 
  jaw 
  equal 
  to 
  or 
  slightly 
  longer 
  than 
  upper 
  jaw 
  ; 
  

   ascending 
  premaxillary 
  process 
  a 
  narrow-based 
  

   spinelike 
  projection 
  ; 
  posterior 
  margin 
  of 
  premaxil- 
  

   lary 
  bone 
  convex 
  or 
  rounded 
  ; 
  origin 
  of 
  first 
  dorsal 
  

  

  