SHEATFISH. 
697 
lateral ethmoid bones. The tongue is wanting. The 
urohyoid bone is of a. singular form. Its anterior, shor- 
ter part (corpus) resembles a parallelopiped. Its posterior 
part is expanded into a rhombic, horizontal disk, with 
the two anterior sides curved upwards and the two 
posterior sharp, but deeply concave. Above this disk, 
and at right angles to it, rises a strong, elevated ridge, 
from the extreme beginning of the bone. 
The form of the dorsal tin is especially characteris- 
tic of the Sheatfish and reminds us strongly of the 
adipose tin in our Salmonoids, resembling an obliquely 
linguiform flap on a fairly narrow stalk, snapped, as 
it were, and bent backwards at the middle. The fin 
lies at a distance from the tip of the snout that mea- 
sures in young specimens about 27 %, in old about 23 
% of the length of the body", or in the former about 
69 % , in the latter about 63 % of the distance between 
the anal fin and the tip of the snout. Its four inter- 
spinal bones coalesce into one supporting bone, the lower 
extremity of which is attached between the tops of the 
fifth and sixth neural spines (belonging to the sixth 
and seventh vertebrae). The first ray is simple but 
distinctly articulated, the posterior two 6 or three rays 
are branched and articulated. 
The anal fin, the length (base) of which usually 
measures more than half (about 53 or 54 % c ) of the 
length of the body, is of almost uniform height through- 
out its length, though its height increases behind in the 
same proportion as the depth of the body decreases. 
Its greatest height (longest ray) is as a rule about 1 / 1G 
— 7,3 of the length of the body, but may sometimes 
rise to about l / a thereof. The fin begins just behind 
the vent with its fairly large genital papilla, at a di- 
stance from the tip of the snout that measures in young 
specimens about 40 — 43 %, in old about 36 % of the 
length of the body. Its last ray is united by a fin- 
membrane to the lower margin of the caudal fin, as 
far as the black colour extends over its base. 
The caudal fin, which during life seems always to 
be directed downwards, sometimes almost at a right 
angle to the longitudinal axis of the body d , may, how- 
ever, be raised and brought without violence into the 
position which it occupies in our figure. In this posi- 
tion, however, the skin forms a protuberance at the 
dorsal margin just in front of the fin. The length of 
this fin generally measures in young specimens about 
11%, in old about 9 — 87 2 % of that of the body; but 
the variations in this respect are quite as great as in 
the case of the height of the anal fin, and in a speci- 
men 1 metre long we have found the length of the 
longest caudal rays to be 147 2 % of that of the body. 
The pectoral and ventral fins are the only ones in 
whose various relations we apparently find perceptible 1 
external differences between the sexes. These fins are 
relatively larger in the males than in the females, to 
judge by our measurements of two males respectively 
about 1 and 2 metres long and two females respectively 
57 7 2 and 737 2 cm. long, our only specimens in which 
the viscera were sufficiently well preserved to enable 
us to decide the sex with certainty. 
The pectoral fins are broad, but obliquely oval. 
Their insertion is partly covered at the upper angle by 
the broad branchiostegal membrane. Their first- (upper- 
most) ray is strong and spinous, though articulated at 
the tip, and forms a weapon which in old specimens 
develops a number of spines on the inside of the outer 
(distal) part. When this weapon is employed, the spi- 
nous ray is erected in an outward direction, a creaking 
sound is heard, a sound which the fish probably uses 
to inspire terror, and the articulation is locked, so that 
the ray cannot be forced back. This is a faculty espe- 
cially common among the fishes of this family, and the 
manner in which this result is attained is as follows. 
The base of the spinous ray is not only furnished as 
usual with two articular knobs — one for each of the two 
articular surfaces which lie on the hind margin of the 
firm osseous connexion between the scapular and cora- 
coid parts of the shoulder-girdle, and of which the outer 
(the one nearer to the clavicle) is an obliquely-set and cir- 
cularly hollowed groove — it is also furnished on the out- 
side with a tumid, pulley-shaped, articular knob, which 
fits into a corresponding ear-shaped groove on the in- 
“ To judge by these changes of growth, as well as by the vomerine cards of teeth, which in the Indian Silurus ivynaadensis (Day, 
Fish. Ind., p. 480, pi. CXI, fig. 6) are separated from each other, this Indian form seems to stand nearer the original type common to both 
species, though we must not forget that the Indian species is known only in specimens at most 12 in. (305 mm.) long. Besides, external 
differences of sex are unknown in these species. 
b In old specimens we have only found two branched rays in the dorsal fin. 
c According to Kroyeu, however, sometimes (in young specimens) 49 or 50 %. 
d This downward curvature of the caudal fin and its union to the anal fin are given by Sagemehl (Morphol. Jalnb., X 1884), p. 5) 
as one of the proofs of the close relationship between the Gymnotoids (the so-called Electric Eels) and the Siluroids. 
