522 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



position on the midventral line ; x the large size and subterminal posi- 

 tion of the mouth ; the development of the sensory canal system 2 of 

 the head, with thin bony septa connected by thin external mem- 

 branes ; 3 the large size and thinness of the scales ; and the correlated 

 position of the anus (immediately before the anal fin) and the num- 

 ber of branchiostegal rays (7). 



44. HYMENOCEPHALUS GRACILIS, new species. 



Type-specimen.— Cat. No. 78227, U.S.N.M., 96 mm. long to end of 

 pseudocaudal, 29 mm. long to anus; dredged by the steamer Albatross 

 at station 5292, in the China Sea off southern Luzon (lat. 13° 28' 45" 

 N.; long. 121° 01' 12" E.) ; depth, 162 fathoms; bottom temperature, 

 52.4° F. 



Fin-rays — first dorsal, II, 10 ; pectorals, 13 ; ventrals, 8. 



The form of the body is entirely similar to that of H. tenuis. It is 

 slender throughout, the depth gradually decreasing toward the end 

 of tail ; the width of the head, equal to the greatest depth of either 

 head or body, is contained twice in length of head. The sides of the 

 head are strongly convex ; the head in cross section is round, instead 

 of rectangular as in H. striatissimus. The snout projects forward be- 

 yond the tip of the premaxillaries a horizontal distance half as long 

 as the pupil ; preocular length of snout, 1.4 in length of orbit, 3.2 in 

 length of head. The orbit is oval in outline; its length is contained 

 1.25 times in the postorbital, or 2.9 times in the entire length of head. 

 The middle of the length of the head is at the hind margin of the 

 pupil. The orbit encroaches medially upon the interorbital, the sides 

 of which, in consequence, are strongly concave ; the least interorbital 

 width is contained about 5 times in the head, being not much more 

 than half the orbital length; least suborbital width, 0.3 length of 

 orbit. The mouth is large and a little oblique; the upper jaw, which 

 extends backward to a vertical intersecting the eye behind the pupil,, 

 is contained 2.25 times in the head. The small teeth are arranged in 

 narrow bands in the two jaws. The preopercular ridge, as in tenuis y 

 is rounded at its angle, not being acutely produced backward as in 

 the subspecies of striatissimus ; the preopercular margin is widely 

 rounded, and but little produced backward. The head, as in tenuis^ 

 is comparatively firm, and the sensory canals, though spacious, are 

 much less developed than in such species as striatissimus ; these 

 canals are covered over by delicate membranes supported by thin 

 bony septa. 



The gill-membranes, free from the sides of the isthmus, extend for- 

 ward to below end of maxillary, where they form a narrow free fold 



1 Somewhat similar and doubtless homologous structures occur In other genera, par- 

 ticular in certain species of Ventrifossa, such as V. nigromarginata (q. v.). 



2 The head is firmer in Hymcnogadus than usual in the other groups. 



8 In this character Hymenocephalus is approcahed by certain species of Ventrifossa. 



