STI!HGE(IN. 



lOoil 



in iituiil)('r Mild lit' t'airU' ei|u;il Icnjitli, soiiietiiiics ex- 

 tending', wlirii l.iid liMck, r.-itlur iR'.-ir, l)u1 iicnit i|iiitc 

 to tlic .■inlri'iiir iii.-iruiii dI the iiioiitli, si'ldoiii indeed In 

 tile IkiIIow ill llie etliiiiiiidal enrtil;ige fur tlie reee|itii>ii 

 of tlie latter. In tlie median line of tlie under surface 

 of tlie snout till re lies in front a sefies of tubereulated 

 bonv jilates in the skin, ]>aired or, at the very lie^'in- 

 iiiiiiT, set ill se\-errtl iri'e^iular rows. Thest; i'()\\s lerini- 

 iiate hehiiid, lietween the middle barbels, in a |>late 

 whieh does not belong exclusively, however, to the skin, 

 but is merely the prominent, though sometimes concealed, 

 head of the vomer (tig. 28."), p. 1046X whose hind ex- 

 tremity, MS we have described above, is embedded in 

 the cranial cai'tilMge and meets the parasphenoid bone, 

 'fliiis we lia\t' hen- a most excellent aiKitomical example 

 of the morphological transition from dermo-ossiticatioiis 

 to parts of the endoskeleton. 



The plate-armour of the head we considenid above, 

 when we selected the Sturgeon as our tv'pe of the ge- 

 neral conformation of these parts within the family. 

 There remains only the opercular apjiaratus, which ex- 

 teriiallv resembles that of the Teleosts, but is charac- 

 terized ])\ the absence of rays in the thick branch iostegal 

 lueiJibrane. The gill-o]jenings extend right across the 

 sides of the body, the branchiostegal inemliranes being 

 united below to the isthmus in a line with the uj)per 

 angles of the gill-ojienings, and separated from each 

 nthei' 1)\ a distance iiMrdly greater in young Sturgeons 

 than the diameter of the eyes, but which widens in 

 older specinu'iis until it is at least half of that between 

 the upper angles of the gill-openings. Each bi-anchio- 

 stegal membrane contains three bones, in a, row one 

 helnw aiiotlier. 'flie uppermost is the o|)erculum, 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 

 i|iiadraut, and with the inferior radius incurvated. At 

 the anterior part of its lower angle, and iinitefl by a 

 tirni suture to its inner surface, is inserted the more 

 elongated suboperculura, 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 s|)ines. The ridge 

 is continued on the lowest bone, explained as an inter- 

 operculum, which has a more quadrilateral form, but 

 is irregularlv incised, with radially disposed ridges on 

 the outer surface. 



The dorsal tin lies fir back, farther in old Sttir- 

 geoiiN tliaii ill \-oiiiig. Tlie distance between its begin- 

 ning and the tip of the snout increases with age from 

 about 111' to (;:) % of the length of the body, its last 

 ray is so small in (j1<1 Sturgeons that its trapezoidal form 

 approaches to the triangular. The ujiper posterior mar- 

 gin is conc'a\c. 'Ihe length of the liase varies between 6 

 and S- % of the length of the body, and the height is 

 about the same, somewhat greater or less. The tirst three 

 or four rays in old Sturgeons are ver\' short, thick, and 

 hai'd, reseml)ling I'ulcra. As a rule, the tirst seven or 

 eight rays are simple, gradiialh' increasing in length, 

 and the ninth or tenth ray is the longest in the tin. 

 The anal tin is opposed to the posterior inirt of tlu' dorsal, 

 and resembles tlu^ latter tin, but has a shorter base 

 — varying between about .'! and 4',., % of the length of 

 the body — and consecpiently a more pointed form. The 

 form and covering of the caudal tin we have already 

 noticed. The upper lobe (tlii' very tip of the tail), which 



Fi^-. 29->. Head lit' till- Slur.i;:i-i>ii. witli nii.iitli iirotnided lilie m proboscis. 



After liKNKrKE. 



in adult specimens (more than '2 dm. in length) has all 

 its upper rays, except the outermost 8 — o, transformed 

 into hard fulcra, is nearly twice as long as the lower. 

 The pectoral tins are set low, almost in the plane of 

 the belly, and liorizontalh-. They are obliquely pointed 

 in form (tig. 2SfS, \>. 1049), with rounded inner jiosterior 

 angle. Their length varies between about 12 and 11 * 

 of that of the body, or between about 22 and 18'., % of 

 the distance between the \entral tins and the tip of the 

 snout, affording an easy distinction between the present 

 species and the Sterlet, which has jjerceptibly longer pec- 

 toral tins. The first ray is considerably stronger than 

 the rest ; but distinct traces in its structure mark it as 

 the i-esult of a coalescence of several (8 — 10) simple and 

 articulated rays. The ventral tins 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 (tig. 289) is less pointed and more 

 traj)ezoidal than that of the pectorals. Tlieir length dur- 



