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Sturgeons of the Genus Seaphirhynchus 181 <br />_Mouth width. —The mouth is Rider in young fish than in old fish, <br />t there: appears to be little change in relative breadth after a bode <br />length of about 250 mm. is reached. Beyond that size the mouth is <br />usually wider in album than in platorynchus. In album the variation <br />is from 8.4 to 9.6 per cent of body length, and the width is more than <br />8.9 per cent in 9 (82 per cent) of 11 specimens; in platorynchus tli;- <br />commarable values are 7.1 to 9.1 per cent. less than 8.7 per ecr,` itt <br />per cent) of 30 examples. Although a reliable char,_ or <br />.,reserved specimens, this measurement is subject to modifica- <br />tion from shrinkalge. <br />Arrangement and positian of barbels. —The most obvious differ- <br />ences between the species of Seaphirhynchus involve the barbels <br />(Fig. 9). In platorynchus the bases of the four barbels typically lie <br />in a straight line, or the outer pair originate slightly ahead of the <br />.rir. so that the line described is concave forward. In contrast, <br />1,11: :_tier barbels are placed behind the inner barbels in album; thus <br />the line through their bases is convex forward. Of 13 examples of <br />album, 11 have the outer barbels well behind the inner <br />whoreas 2 have them only slightly behind. Of 78 spec•ir:.• of <br />r' ;:�:chus examined-for this character, 64 have the barbels in a <br />' ` line, 13 (mostly under 250 mm. long) have the outer barbels <br />o? the inner, and 1 has the outer barbels slightlt- behind the <br />inner barbels. (Forbes and Richardson's figure [1905: fzr ^in_T p. 441 <br />of album fails to show the relationship accurately-.) FurU, it <br />platoryn.chini all of the barbels are farther forward on the loweI- surface <br />of the snout than in album. This difference may be demonstrated <br />—t,:Irisons of the distances from the tip of the snout to the base <br />Cr barbel, which is greater in album (Fig. ;:, or from the <br />inner barbel to tlt • mouth, which is greater in platorynchus (Fig. 4). <br />Except for fish less than about 150 mm. long, in which the values for <br />1 -a4ly measurements are high, these characters show no marked v rict- <br />tiou with size. As may be observed in Figures 3 and 4, there is little <br />overlap between the species in either measurement. Except for a <br />single unusual specimen of platorynchus (308 mm. long - in which <br />the antrr•ior portion of the snout is exceptionally sharp - pointed and <br />]c: tleeued as in al/mm, the measurement from snout to outer barbel <br />is adequate for correct identification. The ratio "snout tip to outer <br />barbel:mouth to inner barbel" provides an effective expre =;yon (Table <br />since it varies from 1.?74 to Q.19 in platorynchus. from ?.?9 to <br />?(> in album. <br />