1058 



SCAXniN AVIAN TISIIKS. 



(lorsocauflal row. The jjlatcs of the vt-iitral lines vary 

 in each row witliiii the jjreabdoiniiial region, as a rule, 

 between 10 and 12; but Kkiivek once found only 8, and 

 iji,L.lEiii)i!(; adduces as many as 14. Behind the vent 

 there generally lie 2 or o pairs in front of the anal 

 fulcrum, or the posterior of these jxiirs coalesces into 

 one plate. Behind the anal fui are svi. 8 — ') (in excep- 

 tional cases only "i) pairs ni' distinct plates in front of 

 rhc lower caudal fulcrum: but to the postei-ior part of 

 the anal tin the same remarks apply as to the corre- 

 sponding part of the dorsal. In the preceding pages 

 we have also considered the transformation of the I'e- 

 maining dermal covering from small scattered spines 

 and their more or less distinct agglomerations, simul- 

 taneously with their adoption of a blunter, more tuber- 

 culate foian. into the diamond-shaped dermal scutes, 

 arranged in obliipie transverse rows, with small rows of 

 tubercles between them, that are especially pi'ominent 

 on the dorsal sides above the lateral lines. ()n the fins 

 and their rays the small, sharp spines are persistent; 

 Imt on tlie upper part of the caudal fin, on each side 

 (if the continuation of the s])inal column within this 

 fin, elongated, diamond-shaped, and smooth bony plates 

 are developed, arranged in oblique rows running from 

 below and in front upwards and backwards. 



The hind part of the head is rathei- terete, con- 

 stricted above at the gill-openings and eyes, with more 

 or less depressed interorbital space. A striking diffe- 

 rence from the head of the Teleosts is that the operctda 

 do not entirely cover the gill-openings, the gills of the 

 Sturgeon being consequently laid bare to some extent 

 behind. In front the head is de])ressed and of varying 

 length, tapering in a ]iyramidical form, with the superior 

 surface of the snout more convex than the inferior and 

 the other two sides narrowing S(jmetimes to an edge. 

 In young Sturgeons the relative length of the snout, as 

 we have mentioned above, is considerably greater than 

 in old, varying witli age from at)iiut 14 % at least to 

 about 8^/^ % of the length of the body, or from 53% 

 (according to Kroyek sometimes 57 %) to 42 % of that 

 of the head, so that the entire length of the head is also 

 comparatively less in the latter. The postorbital length 

 of the head, on the other hand, I'elativcly remains al- 



nu)st unaltered during growth, being about ' ,,|"ofthe 

 length of the body. The eyes are small and somewhat 

 oval. Their longitudinal diameter, which shows relative 

 diminution during the growth of the fisji, varies between 

 10 and 7 % of the length of the head, between 19 and 

 14 "-. of the length of the snout, or between 37 and 22 % 

 of the interoi-bital width''. The interorbital space, which 

 is ciiiivcN. liut medially more or less depressed, grows 

 narrower in proportion to the general growth, varying 

 between about 7' ., and 6' ., % of the length of tlie body, 

 but wider in proportion to the length of the head, 

 varying between about 27 and 33 % thereof. In front 

 of the (;yes lie the large nostrils — comparatively larger, 

 however, in young Sturgeons than in old — the posterior 

 in each pair being the larger, an obliquely or even a 

 transversely set ellipse, the length of which is some- 

 times greater than the diameter of tlie eyes, and situated 

 low down; the anterior more extended in the longitu- 

 dinal direction of the body, and set nearer to the level 

 of the forehead. They are separated by a thin (in old 

 sjjecimens rather broad) dermal ridge. The shortening 

 of the snout during the growth of the fish is accom- 

 plished at the expense of its anterior part (the ro.stral 

 cartilage), the distance from the anterior nostril to the 

 ti]) of the snout being sometimes reduced with age 

 from 47 to 28 % of the length of tlie head. 



The under surface ot the head, like tliat of the 

 belly, is rather plane. Behind the perpendicular from 

 the centre of the eyes is situated the comparatively 

 small mnutlr. Owing to its structure, which we have 

 described above, it may be protrudetl (in a downward 

 direction), so as to form a square tube, rounded, how- 

 ever, at the corners (fig. 292). The toothless jaw-margins 

 are rounded. The lips are tumid, on the upper jaw 

 rather uniforndy, but with a median indentation, at the 

 corners of the mouth in a bulging form, on the lower 

 jaw in the form of two fleshy folds, which ai'e closely 

 applied to each other from the sides, but distinct, the 

 jaw itself being medially nidged. Half-way between the 

 tip of the snout and the mouth — sometimes, in young 

 Sturgeons, distinctly nearer to the latter, sometimes, in 

 old, nearer to the former' — hangs the transverse series 

 of barbels characteristic of the Sturgeons. These are 4 



" Varying, according U> onr nii'iisurcniPiils, between about 10'., and !)' ., %. 



'' For the vertical diameter of tlie eyes the corresponding percentages are respectively 8 and .'>' ,, Hi and 12. 30 and lH. 

 •■ One of the niosl reliable distinctions from the Hansen (Acipenser huso) and the OssAr (Tok-, Ar. CulcU-nstadtH), whicli have a 

 oader mouth. 



■' The distance between the barbels and the tip of the snout measures 04—44% of Ibat- between the uionib and (he same point. 



