508 



BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



The dorsal and ventral contours are long, even curves ; the base of 

 the first dorsal is scarcely oblique; the dorsal contour of the snout 

 is but little (or scarcely) concave. Greatest depth of body, 2.2 

 (1.8 to 2.3) in length of head to end of membrane at angle of 

 opercle ; the greatest width of the body or head is about equal to the 

 depth below the origin of the lateral line, and about equal to the 

 depth of the head at the vertical passing through the hind margin 

 of the orbit. The form of the head and trunk is shown in figure 27, 

 in which the width of the body is abnormally contracted, owing 

 to the imperfect preservation of the type-specimen. The length 

 of the snout is unusually variable in this species, so variable that 

 Smith and Radcliffe separated out the short-snouted specimens 

 as a distinct "form," alpha (figure 29). That two distinct groups 

 are not represented may readily be seen by an examination of the 

 following table: 



Table of measurements of snout in C. commutabilis. 



1 Type-specimen. 



2 Measured into length of head. 



» Measured at end of ethmoid portion of infraorbital ridge, into length anterior to that point. 



The ridges of the head are covered by a single series of strong 

 scales, excepting the infraorbital ridge behind the middle of the eye, 

 where the scales are in two series. The occipital ridges diverge a 

 little toward both ends ; the least distance between them is contained 

 2.2 (1.8 to 2.2) times in the interorbital width. The rounded margin 

 of the preopercle is scarcely denticulate; the lower ventral angle of 

 the subopercle is produced backward into a slender flap. The orbit 

 is large, as usual in this group of species, but widely variable in 

 size, being contained 3.9 (3.33 to 3.95) times in entire length of head 

 and 1.25 (1.15 to 1.3) times in postorbital length of head. The 

 variation in the size of the orbit is not correlated with the size of the 

 specimens; the extremes are joined together by intermediate sizes. 

 Least interorbital width, 1.38 (1.33 to 1.6) in postorbital length of 

 head; least suborbital width, 2.9 (2.2 to 2.8). The upper jaw 

 extends backward from below the anterior nostril (or immediately 

 behind that vertical) almost (or quite) to below the hind margin 

 of the orbit; the length of the upper jaw is contained 3.75 (3.35 to 

 4.0) times in the head; barbel, 3.6 (3.5 to 6.0) in postorbital. Six 

 branchiostegals ; gill-membranes with a free fold across the isthmus. 



