980 



SCANDINAVIAN FISHES. 



The greatest depth varies between about 19 and 

 22 % of the length of the bod)' to the end of the middle 

 caudal rays: and the greatest tiiifkness between about 

 4') % (in small sjiorimcns) and ')l;l "o of the greatest 

 depth. 



The length of the head (measured as above, in the 

 G^vyniads and Herring, from the articular knobs of the 

 maxillaries) varies lietween about 22',,, and 21' ., % of 

 that of the bodj'. The diameter of the ej'es, which 

 are circular, measures (in Pilchards 81 — 211 mm. long) 

 between .')() and 2;^ "*, 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 

 80 % of the length of the iiead. The nostrils — the 

 .-interior in each ])air is merely a niiri-ow 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. 



ti]) 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 '/j to ST'/, % of that of the 

 iiead, and their breadth varies between about 36 and 

 32 % of their length. The length of the cheek (from 

 the hind extremity of the maxillarj' 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 symjihysis 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, tig. 247) the 



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 meirdarane 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 jiart of the scapular region is en- 

 tirely co\ered 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 o])erculum) 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 ujiper |)Osterior 

 angle rounded, the posterior side rather convex, and the 

 inferior side straight. The last-mentioned side (the sub- 

 opercular suture) measures about 22 or 21'/., % of the 

 length of the head. The form of the subopercuhun 

 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 ]iosterior 

 branchiostegal rays as in the Herring, these ra}'s form- 

 ing a rectangular sinus \vith the inferior margin of the 

 suboperculum. On opening the operculum we find that 

 the l)lack jiosterior 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 suboj^er- 

 culum \\'ith the horizontal arm, the margin of which 

 consists of the above-mentioned process (answering to 

 df in fig. 238, p. 950). The gill-rakers are long and 

 fine, with dense and regular lateral spines of small size. 

 On the fir.st 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 

 extremel)' feeble, being usually confined to a few teeth 

 at the tip of the lower ja-^v and a crenelation of the 



Cf. MoKEAu's dofiiiitioii of the genus Sardinella (1. 



450): "Ceinturc sc.T]iulaire a liord anterieur vertical' 



