Gila nigrescens × Rhinichthys cataractae: KU 4253 (a male, 60.6 mm. in standard length), from New Mexico, Bernalillo County, Rio Grande 12 mi. S Bernalillo on U. S. Highway 85 (Corraleo Bridge). Compared in Table 4 with six specimens of G. nigrescens: KU 4251, 4254, and 4262 (63.1-72.4 mm. in standard length, mean 66.4 mm.); and with five specimens of R. cataractae: KU 4248, 4258, and 4264 (55.6-65.0 mm. standard length, mean 59.5 mm.). Comparative material was taken at the same locality as KU 4253 and at nearby localities in the Rio Grande.
The hybrid is intermediate in almost all of the features in which the parental species differ from each other. For six of the characters included in Table 4, the hybrid index is 49.7 per cent, when Gila is assigned the value 0 (height of dorsal fin and numbers of fin rays and teeth excluded). There is no enlargement of the head in KU 4253, such as was found in Gila orcutti × Siphateles mohavensis (Hubbs and Miller, 1943:373), Chrosomus erythrogaster × Notropis cornutus frontalis, and C. erythrogaster × Semotilus atromaculatus. The height of the dorsal fin, which Hubbs and Miller (loc. cit.) found to be extreme in G. orcutti × S. mohavensis, exceeds the average for the parental species in G. nigrescens × R. cataractae also; but, dorsal fins as high as that of the hybrid were found in some individuals of both parental species. In R. cataractae, all fins are more rounded and more expansive than in G. nigrescens, and fins other than the dorsal have an intermediate size in the hybrid. This intermediacy has doubtful significance, because fin-size in Rhinichthys varies greatly with body-size, sex, and probably with the state of sexual development. Rhinichthys matures at smaller size than Gila, and never becomes so large as that species.
Gila nigrescens and R. cataractae differ strikingly in features involving the snout and mouth, and these differences provide the most conclusive evidence that KU 4253 is a hybrid of these species. The projecting, fleshy snout of R. cataractae is bridged to the ventral mouth by a frenum that is approximately 3 mm. wide in specimens 60 mm. in standard length. In Gila, the snout does not project beyond the mouth, which is oblique, lacks a frenum, and is larger than in Rhinichthys. The snout of the hybrid projects less than in R. cataractae and is bridged to the upper lip by a frenum 1.7 mm. wide. The mouth of the hybrid is intermediate in size, obliquity, and thickness of the lips. Rhinichthys has barbels, Gila lacks them, and the hybrid has one vestigial barbel, on the right side. The lower surface of the head of Rhinichthys is broad and flattened, with pronounced rugosity on the gular area and isthmus. In Gila the underside of the head is convex, with comparatively smooth membranes; the hybrid is intermediate, but tends toward Gila.
Table 4. Comparisons of One Specimen of Gila nigrescens × Rhinichthys cataractae with Specimens of the Parental Species (means are above, ranges in parentheses below)
| Gila nigrescens | KU 4253 | Rhinichthys cataractae | |||
| Standard lengths | 66.4 (63.1-72.4) | 60.6 | 59.5 (55.6-65.0) | ||
| 282 (277-290) | 281 | 281 (273-293) | ||
| 063 (063-065) | 054 | 044 (041-047) | ||
| 083 (081-085) | 092 | 106 (099-113) | ||
| 225 (212-238) | 234 | 221 (206-234) | ||
| 140 (134-142) | 135 | 131 (127-136) | ||
| 081 (079-085) | 076 | 066 (064-069) | ||
| 060 (058-064) | 045 | 036 (034-038) | ||
| Upper jaw | protractile | non-protractile | non-protractile | ||
| Number scales in lateral line | 60 (58-63) | 63 | 65 (63-67) | ||
| Anal fin-rays | 8 (7-8) | 7 | 7 (7) | ||
| Pelvic fin-rays | 9 (9) | 8 | 8 (8-9) | ||
| Pectoral fin-rays | 16 (16-18) | 16-15 | 13 (13-14) | ||
| Pharyngeal teeth | 2,5-4,2 | 2,5-4,2 | 2,4-4,2 |
Table 5. Comparisons of One Specimen of Notropis v. venustus × Notropis whipplei with Specimens of the Parental Species, and with N. lutrensis × N. v. venustus. Measurements (lengths and depths) Are Expressed as Thousandths of Standard Length (means above, ranges in parentheses below)
| Notropis whipplei | KU 3516 | Notropis venustus, KU 3510 | Notropis venustus, from Gibbs (1957a) | Notropis lutrensis × N. venustus | |
| Standard length | 50.6 (45.0-54.0) | 47.8 | 47.3 (44.5-49.6) | 44.7 (43.3-47.3) | |
| Predorsal length | 525 (513-535) | 523 | 534 (519-547) | 523 | 532 (528-538) |
| Dorsal origin to caudal base | 497 (493-502) | 508 | 497 (478-504) | 496 | 508 (502-514) |
| Prepelvic length | 505 (498-518) | 492 | 505 (500-510) | 499 (486-517) | |
| Head-length | 257 (250-262) | 255 | 261 (256-267) | 260 | 263 (261-267) |
| Caudal peduncle-length | 217 (211-220) | 221 | 224 (213-230) | 224 (214-231) | |
| Caudal peduncle-depth | 110 (106-116) | 119 | 127 (124-133) | 125 | 126 (122-131) |
| Head-depth | 170 (167-173) | 182 | 186 (182-190) | 190 (189-192) | |
| Snout-length | 079 (076-083) | 079 | 080 (072-083) | 081 (078-082) | |
| Eye-diameter | 069 (063-078) | 069 | 070 (066-072) | 073 [A] | 070 (068-074) |
| Postorbital length, head | 112 (108-115) | 115 | 116 (112-120) | 117 (115-120) | |
| Upper jaw, length | 078 (076-081) | 077 | 081 (076-082) | 079 | 077 (076-081) |
| Body depth | 239 (233-248) | 253 | 278 (261-288) | 274 | 282 (275-294) |
| Lateral-line scales | 36-37 | 36 | 36-38 | 36.5 (34-39) | |
| Scales above lateral-line | 13 | 14 | 15 | 15 (13-16) | |
| Anal fin-rays | 9 | 9 | 8 | 8 (7-8) | |
| Pectoral fin-rays | 14 (14-15) | 14-14 | 15 (14-16) | 14.2 (12-17) | |
| Caudal spot | Absent | Present | Present | Present | Present |
| Vertebrae | 37-38 | 38 | 37 |
[A] Orbital diameter.
The air bladder of KU 4253 is nearly as large as in Gila, and much larger than the degenerate air bladder of R. cataractae. Although the hybrid appears to be male, the gonads (especially the right one) are poorly developed. The hybrid is intermediate in curvature of the lateral line, which is nearly straight in Rhinichthys and strongly decurved in Gila.
Specimen No. 4253 is mostly pallid, resembling Gila much more than Rhinichthys in pigmentation. A mid-dorsal dark streak is conspicuous in the hybrid, especially anteriorly, but is less intense than in Gila. Rhinichthys lacks a well-developed dorsal stripe. The preorbital and suborbital areas are more heavily pigmented in the hybrid than in Gila, but not nearly so dark as in Rhinichthys. The hybrid has a faint dark basicaudal spot that is variably developed in Rhinichthys but absent in Gila.