_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_ | | _Rhinichthys_ | _nigrescens_ | KU 4253 | _cataractae_ ----------------------+--------------+-----------------+----------------- Standard lengths | 66.4 | 60.6 | 59.5 | (63.1-72.4) | | (55.6-65.0) | | | Head-length / | 282 | 281 | 281 Standard length | (277-290) | | (273-293) | | | Orbital length / | 063 | 054 | 044 Standard length | (063-065) | | (041-047) | | | Snout-length / | 083 | 092 | 106 Standard length | (081-085) | | (099-113) | | | Dorsal fin-height / | 225 | 234 | 221 Standard length | (212-238) | | (206-234) | | | Postorbital length / | 140 | 135 | 131 Standard length | (134-142) | | (127-136) | | | Distance from tip of | | | mandible to tip of | | | maxillary / | 081 | 076 | 066 Standard length | (079-085) | | (064-069)