296 



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



The scales have the same general shape and structure as in the preceding species ; the lateral 

 line heing also nearer the ventral than the dorsal outline. 



The upper regions are of a greyish azure ; the inferior regions heing of a dull silvery white 

 tint, with numerous black dots scattered all over the hack, sides, and belly. The fins are 

 unicolor, the inferior ones much lighter than the others. 



List of specimens. 



SIBOMA, Girard. 



GEN. CHAR. Body stout and somewhat compressed, covered with large scales. The caudal is crescent-shaped posteriorly ; 

 the origin of the ventrals is situated a little posteriorly to the anterior margin of the dorsal, or immediately under it. The head 

 is rather small, sloping towards a wedge-shaped srout, superiorly convex. The mouth is of moderate or small size, horizontal, 

 terminal, with even jaws. No barbels. Eye below the medium size. Isthmus rather narrow. Pharyngeal bones stout, 

 expanded upon their convexity, with the inferior branch short and bent inwardly so that its extremity is directed outwardly, 

 whilst the upper branch is slightly bent inwards. The teeth are large and very compressed, and terminated by a slender hook. 

 They are of the raptatorial kind, of the hooked type, without grinding surface, properly so called, but instead of a sharp edge 

 along the inner margin of the teeth, a blunt and narrow ridge may be observed. They are disposed upon a double row of one 

 and two and four and five, as follows : 1 | 4 5 | 2. 



SYN. Siboma, GRD. in Proc. Acad. Nat. Sc. Philad. VIII, 1856, 208. 



The general aspect of this genus resembles more that of Tigoma than any other of the same 

 family. The chief difference between the two genera consisting in the position of the ventrals 

 in reference to the dorsal. 



1. SIBOMA CRASSICAUDA, Grd. 

 PLATE LXIY, FIGS. 14. 



SPEC. CHAR. Body somewhat elongated, rather thick. Peduncle of the tail very stout. Head contained four times and a 

 half in the total length, its profile is very much inclined, and the snout sub-pyramidal. Posterior extremity of maxillar bone 

 extending to a vertical line drawn midway between the nostrils and the orbit. Eye rather small, sub-circular ; its diameter 

 entering nearly six times in the length of the side of the head. Anterior margin of dorsal fin a little nearer the extremity of the 

 snout than the fork of the caudal. Light purplish brown above ; yellowish beneath. 



SYN. Lavinia crassicauda, B. & G. in Proc. Acad. Nat. Sc. Philad. VII, 1854, 137. 

 Siboma crassicauda, GRD. in Proc. Acad. Nat. Sc. Philad. VIII, 1856, 208. 



The body in this species is very stoutly built, maintaining its depth posteriorly. The peduncle 

 of the tail is very much developed. The greatest depth is nearly equal to the length of the 

 head, which is contained four times and a half in the total length. The head itself is propor 

 tionally large, of moderate size ; its upper surface is very much inclined forwards from the 

 occiput to a nearly wedge-shaped or sub-pyramidal snout. The gape of the mouth is moderate. 

 The eye is sub-circular, and its diameter is contained about six times in the length of the side 

 of the head. The caudal fin is but moderately emargiiiated posteriorly, and resembles more 

 a crescent on that margin than a fork. The dorsal and anal are well developed, sub-trapezoid 

 in shape ; the latter being somewhat smaller than the former, and its anterior margin equi- 



