288 



U. S. P. R. R. EXP. AND SURVEYS ZOOLOGY GENERAL REPORT. 



is a little higher than long, with its upper margin slightly concave. The anal fin resembles 

 the dorsal in form and proportions, being slightly convex exteriorly and deeper than long. Its 

 anterior margin is situated a little Backwards to the posterior margin of the dorsal. The caudal 

 is deeply furcated ; its lobes being symmetrical. The developed rays of these three fins are 

 bifurcated twice upon their length. The ventrals are sub-oval ; their insertion is placed some 

 what in advance of the dorsal, and when bent backwards their tip reaches the vent and 

 occasionally also the anterior margin of the anal fin. The pectorals are sub-triangular, 

 tapering ; their insertion is close to the head, rather more towards the inferior aspect than on 

 the sides ; their extremity does not reach the insertion of the ventrals, and consequently not the 

 anterior margin of the dorsal. 



D 3, 8 ; A 3, 8 ; C 8, 1, 10, 11, 1, T ; V 1, 8 ; P 16. 



The vent is situated close to the anal fin, and nearer the tip of the lower lobe of the caudal 

 than the extremity of the snout. The scales, as a general rule, are rather small, thin, and 

 sub-circular. The smallest ones occur between the dorsal fin and the occiput, and especially 

 upon the latter region. From the middle of the flanks downwards they increase in size, being 

 slightly imbricated till near the abdomen, whence they diminish on to the belly, where they 

 lose again their imbrication. Radiating grooves affect the posterior section or half of the scale 

 only. The lateral line undergoes a gentle downwards curve upon the middle of the abdomen, 

 where it is nearer the abdominal than dorsal outline, passing through the area of the largest 

 scales, being very conspicuous from head to tail. 



The color is of a uniform yellowish brown tint on the head, and along the dorsal region 

 covered with the smallest scales. The sides and belly are shining silvery white. The fins 

 being yellowish, the inferior ones lighter than the upper. 



List of specimens. 



TIGOMA, Girard. 



GEN. CHAR. Bears a general resemblance to Gila, from which it differs by a much smaller mouth, and larger scales, and the 

 scales of the dorsal region, though smaller than on the sides, are not so disproportionate as in the genus just alluded to. The 

 ventrals are inserted in advance of the anterior margin of the dorsal, and the caudal fin is furcated. It is not deemed superfluous 

 to state that there are no barbels at the angle of the mouth ; and a characteristic of this genus, as distinct from Gila, consists in 

 its terminal mouth and even jaws. The pharyngeal bones resemble very much those of Gila, but the inferior limb is not quite 

 so long, the convexity not quite so expanded, and the upper limb more curved inwards. As to the teeth, it requires a minute 

 observation to dete?t a generical difference ; they are more compressed and more strongly hooked ; otherwise, of the raptatorial 

 kind, of the hooked type, either without a grinding surface or else provided with a grinding surface more or less developed. 

 They are disposed in the following wise : 2 | 55 | 2 ; 2 | 4 5 | 2, or 1 | 4 5 | 2, besides some intermediate formulae. 



SYN. Tigoma, GRD. in Proc. Acad. Nat. Sc. Philad. VIII, 1856, 205. 



Some of the species of this genus we had formerly included in other genera, arid now we add 

 a few more hitherto undescribed. 



