■980 
SCANDINAVIAN FISHES. 
The greatest depth varies between about 19 and 
22 % of the length of the body to the end of the middle 
caudal rays; and the greatest thickness between about 
45 % (in small specimens) and 59 % of the greatest 
depth. 
The length of the head (measured as above, in the 
Gwyniads and Herring, from the articular knobs of the 
maxillaries) varies between about 22 x / 2 and 21 1 / 2 % of 
that of the body. The diameter of the eyes, which 
are circular, measures (in Pilchards 81 — 211 mm. long) 
between 30 and 23 %, and the interorbital width (at 
the middle of the eyes) 20 — 24 %, of the length of the 
head. The length of the snout is equal in small Pil- 
chards to the diameter of the eyes; in adult specimens 
it shows individual variations between about 26 and 
30 % of the length of the head. The nostrils — the 
anterior in each pair is merely a narrow slit — lie 
nearer to the tip of the snout than to the eyes. The 
Fig. 247. Head of a Pilchard, with feebly developed covering of 
adipose membrane. Natural size. 
tip of the mouth, which is formed by the margins of 
the intermaxillaries, is incised at an oblique angle, 
owing to the position occupied by these bones with re- 
spect to each other. The length of the maxillaries, 
which extend back to about a line with the anterior 
margins of the pupils, decreases in the above-mentioned 
specimens from about 39 l f 2 to 3 7 1 /' 2 % of that of the 
head, and their breadth varies between about 36 and 
32 % of their length. The length of the cheek (from 
the hind extremity of the maxillary to the posterior 
margin of the preoperculum) is much greater, as in the 
Herring, than its height below the eyes. The length 
of the lower jaw, the symphysis of which shows hardly 
any thickening, decreases in Pilchards of the above 
lengths from about 55 to 48 % of that of the head. 
The cheeks and opercula vary in appearance according 
to the density of the adipose membrane on the head. 
When this membrane is thin (shrunken, fig. 247) the 
a Of. Moreau’s definition of the genus Sardinella (1. c., p. 450): 
striation both of the lower anterior part of the oper- 
culum and of the preoperculum and suborbital bones 
consists of distinct ridges and grooves. But in other 
instances, when the adipose membrane is thicker (fig. 
246), its ducts with their numerous ramifications occupy 
the grooves, and render these as well as the ridges less 
distinct. The upper part of the scapular region is en- 
tirely covered with this growth of adipose membrane, 
the elevated pores granulating the upper surface of the 
head. Measured in an horizontal direction the postorbital 
length of the head (from the apparent hind margin of 
the eye to the middle of the posterior opercular margin) 
increases with age from about 41 — 46 % of the entire 
length of the same; measured in an oblique direction 
(from the middle of the hind margin of the eye to the 
lower posterior angle of the operculum) it varies between 
about 47 and 53 % of the latter length, and is some- 
what more, even in young specimens, than one-fourth 
of the distance between the dorsal fin and the articular 
knobs of the maxillaries. The operculum forms an al- 
most rectangular parallelogram, with the upper posterior 
angle rounded, the posterior side rather convex, and the 
inferior side straight. The last-mentioned side (the sub- 
opercular suture) measures about 22 or 2 1 1 / 2 % of the 
length of the head. The form of the suboperculum 
varies, its breadth (height) being about one-third of its 
length in young specimens, about one-half of the same 
in adult Pilchards. The preoperculum is broad, with 
rounded, rectangular corner. The interoperculum is 
narrow, and has the same relation to the posterior 
bran chi ostegal rays as in the Herring, these rays form- 
ing a rectangular sinus with the inferior margin of the 
suboperculum. On opening the operculum we find that 
the black posterior limit of the pharynx and the branchial 
cavity is rectangular®, corresponding to the form of the 
operculum itself, so that the posterior margin of the 
latter coincides with the vertical arm of the clavicular 
angle, and its inferior margin as well as the suboper- 
culum with the horizontal arm, the margin of which 
consists of the above-mentioned process (answering to 
cl p in fig. 238, p. 950). The gill-rakers are long and 
fine, with dense and regular lateral spines of small size. 
On the first branchial arch we have counted 110 gill- 
rakers, 67 on the lower (horizontal) part of the arch, 
and 43 on the upper. The dentition of the mouth is 
extremely feeble, being usually confined to a few teeth 
at the tip of the lower jaw and a crenelation of the 
Ceiuture scapulaire a borcl anterieur vertical”. 
