STURGEON. 
1059 
in number and of fairly equal length, sometimes ex- 
tending, when laid back, rather near, but never quite 
to the anterior margin of the mouth, seldom indeed to 
the hollow in the ethmoidal cartilage for the reception 
of the latter. In the median line of the under surface 
of the snout there lies in front a series of tuberculated 
bony plates in the skin, paired or, at the very begin- 
ning, set in several irregular rows. These rows termi- 
nate behind, between the middle barbels, in a plate 
which does not belong exclusively, however, to the skin, 
but is merely the prominent, though sometimes concealed, 
head of the vomer (fig. 285, p. 1046), whose hind ex- 
tremity, as we have described above, is embedded in 
the cranial cartilage and meets the parasphenoid bone. 
Thus we have here a most excellent anatomical example 
of the morphological transition from der mo-ossifications 
to parts of the endoskeleton. 
The plate-armour of the head we considered above, 
when we selected the Sturgeon as our type of the ge- 
neral conformation of these parts within the family. 
There remains only the opercular apparatus, which ex- 
ternally resembles that of the Teleosts, but is charac- 
terized by the absence of rays in the thick branchiostegal 
membrane. The gill-openings extend right across the 
sides of the body, the branchiostegal membranes being 
united below to the isthmus in a line with the upper 
angles of the gill-openings, and separated from each 
other by a distance hardly greater in young Sturgeons 
than the diameter of the eyes, but which widens in 
older specimens until it is at least half of that between 
the upper angles of the gill-openings. Each branchio- 
stegal membrane contains three bones, in a row one 
below another. The uppermost is the operculum, which 
is naked throughout the greater part of its extent, and 
shows the same radial striation and thimble-like granu- 
lation of the surface as the dermal scutes. In form it 
resembles a sector of a circle, somewhat greater than a 
quadrant, and with the inferior radius incurvated. At 
the anterior part of its lower angle, and united by a 
firm suture to its inner surface, is inserted the more 
elongated suboperculum, which lies across the body, 
and in old Sturgeons is mostly covered by the skin, but 
on the outside has a high ridge, above sending out 
through the skin a few granulated spines. The ridge 
is continued on the lowest bone, explained as an inter- 
operculum, which has a more quadrilateral form, but 
is irregularly incised, with radially disposed ridges on 
the outer surface. 
The dorsal fin lies far back, farther in old Stur- 
geons than in young. The distance between its begin- 
ning and the tip of the snout increases with age from 
about 62 to 69 % of the length of the body. Its last 
ray is so small in old Sturgeons that its trapezoidal form 
approaches to the triangular. The upper posterior mar- 
gin is concave. The length of the base varies between 6 
and 8 % of the length of the body, and the height is 
about the same, somewhat greater or less. The first three 
or four rays in old Sturgeons are very short, thick, and 
hard, resembling fulcra. As a rule, the first seven or 
eight ra}'s are simple, gradually increasing in length, 
| and the ninth or tenth ray is the longest in the fin. 
The anal fin is opposed to the posterior part of the dorsal, 
and resembles the latter fin, but has a shorter base 
— varying between about 3 and 4 '/ 2 % of the length of 
the body — and consequently a more pointed form. The 
form and covering of the caudal fin we have already 
noticed. The upper lobe (the very tip of the tail), which 
Fig. 292. Head of the Sturgeon, with mouth protruded like a proboscis- 
After Benecke. 
in adult specimens (more than 2 dm. in length) has all 
its upper rays, except the outermost 3 — 5, transformed 
into hard fulcra, is nearly twice as long as the lower. 
The pectoral fins are set low, almost in the plane of 
the belly, and horizontally. They are obliquely pointed 
in form (fig. 288, p. 1049), with rounded inner posterior 
angle. Their length varies between about 12 and 11 % 
of that of the body, or between about 22 and 1 8 '/ 2 % of 
the distance between the ventral fins and the tip of the 
snout, affording an easy distinction between the present 
species and the Sterlet, which has perceptibly longer pec- 
toral fins. The first ray is considerably stronger than 
the rest; but distinct traces in its structure mark it as 
the result of a coalescence of several (8 — 10) simple and 
articulated rays. The ventral fins are inserted behind 
the middle of the body, at a distance from the tip of 
the snout measuring about 54 — 59 % of the length of the 
body. Their form (fig. 289) is less pointed and more 
trapezoidal than that of the pectorals. Their length dur- 
