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<br />:::30 <br /> <br />II ubbs and Miller <br /> <br />type (X. texanus) has probably increased the incidence of hybridiza- <br />tion with the parental species (of Catostomus) that haNe remained <br />abundant. <br />In being intermediate in most characters between their par- <br />tlcular parental species, the newly found catostomid hybrids con- <br />f,>rm to previously studied interspecific fish hybrids. On the scale <br />grading from 0 for the species of Catostomus involved to 100 for Xyr- <br />a uchen texanus, the hybrid indices for the C. latip~innis X X. <br />t.xanus hybrids vary from 20 to 70 and average 45 for the three <br />distinctive counts combined; they vary from -9 to 91 .and average <br />4'~ for the nine major differences in measured proportions; and they <br />vary from -2 to 81 and average 43 for counts and proportions eom- <br />btned. For the six C. insignis X X. texanus hybrids, the indices <br />v;uy from 15 to 40 (average, 30) for the three counts; from 24 to 86 <br />(li7) for the eight major differences in proportions; and from 24 to 72 <br />(D7) for the counts and proportions combined (Table VIII). The <br />aberrant average indices for certain specimens are primarily a con- <br />s(~quence of the extreme value of the index representing the inter- <br />pdvic space. 'When that index is deleted, the average indices for <br />measurements of each individual of the C. latipinnis X X. texanus <br />combination vary from 25 to 72 and average 47, and for the counts <br />and measurements together they vary from 24 to 66, with the same <br />a \"erage. <br />The lower number of dorsal rays in the parental species of Cato- <br />stnm71S appears to be imperfectly dominant over the higher number <br />in X yrauchen texanus. <br />Chasmistes and Xyrauche11:, though distinctive, are probably <br />close relatives of Catostomus, being adaptations respectively to <br />lal~ustrine and to large, swift river habitats. All these genera are <br />re/erred to the tribe Catostomini. Hybridization in this as in sev- <br />eral other families therefore does not transgress tribal limits. The <br />mi>re heterogenetic crossings indicated for the Cyprinidae suggest <br />that group ranking in that family may have been too high. It <br />would be inconsistent with taxonomic systems to deny specific or <br />even generic rank to forms merely because they hybridize. Cross- <br />ability is only one of the criteria to be considered. <br />The new catostomid hybrids may well have resulted from the <br />cll ance mixing of ova and spermatozoa in stream currents. This <br /> <br /> <br />Hybridization between Catostomus and Xyrauchen <br /> <br />TABLE VIII <br />HYBRID INDICES FOR CATOSTOMUS X XYRAUCHEN HYBRIDS <br />The indices are computed from the data presented or summarized in Tables II-V <br />and VII. Each value indicates the position of the hybrid on a scale of 100 units con- <br />necting the means for the Catostomus specimens, set at 0, with those for the Xyrau- <br />chen specimens, set at 100; a value of 50 denotes precise intermediacy. <br /> <br />9131 <br /> <br /> C. latipinnis X X. texanus C. insignis X <br /> X. texanus <br /> (8 specimens) (6 specimens) <br />Count or measurement I 379-442 <br /> , <br /> 101.7 256 mm. (6 Mean Range Mean <br /> mm. mm. speci- <br /> mens) <br /> - ---- - <br />Lateral-line-scale count . . . . . . . . . . 87 122 -28-87 70 7-67 37 <br />Dorsal-ray count. . . . . . . . . . . . . . . . -10 34 -10-78 23 - 5-48 19 <br />Gillraker count . . . . . . . . . . . . . . . . . 30 49 35-54 46 13-71 34 <br /> - - <br />A vemge for counts. . . . . . . . . . . . 36 68 20-70 45 15-40 30 <br /> -~ ----- - <br />Body depth . . . . . . . . . . . . . . . . . . . . 42 47 4-70 41 .. . .. . <br />Depth through pectoral insertion . . 50 30 Hi-62 36 .. . .. . <br />Caudal-peduncle depth 63 51 29-69 46 7-47 21 <br />......... . <br />Snout length . . . . . . . . . . . . . . . . . . . . . .. . .. . . . 47-100 70 <br />Eye diameter . . . . . . . . . . . . . . . . . . . . . .. . .. . . . -40-200 107 <br />Lips, overall length (A) ... . . . . . . . 37 33 18-78 38 42-183 118 <br />Gape width (B) .... . . . . . . . . . . . . . 93 32 - 6-85 45 .. . .. . <br />Separation, lower lips. . . . . . . . . . . . 12 24 -7-64 22 11-175 76 <br />Gillraker, length of longest . . . . . . . 37 13 12-246 84 .. . .. . <br />Dorsal fin, depressed length . . . . . . . . " . .. . . . 0-91 47 <br />Dorsal fin, length of base . . . . . . . . 47 I 76 41-105 68 -4-96 43 <br />Interpectoral space . . . . . . . . . . . . . . . . .. . .. . . . 5-135 57 <br />Interpelvic space . . . . . . . . . . . . . . . . 40 126 - 283-253 -5* .. . " . <br /> - - <br />Average for measurements. . . . . 47 48 -9-91 42 24-86 67 <br />Without interpelvic space. . . . 48 38 25-72 47 .. . .. . <br /> - - - <br />Average, connts and measure- 57 <br />ments . . .................... . 44 53 -2-81 43 24-72 <br />Without interpelvic space. . . . 44 46 24-66 47 .. . .. . <br />Ratio B/ A (Fig. 3) ... . . . . . . . . . . . 52 22 13-45 30 .. . .. . <br /> <br />* Seven specimens. <br />