Page:Encyclopædia Britannica, Ninth Edition, v. 12.djvu/666

648 648 few branches (d) only, the remaining branches (c) being fixed to the basal cartilage. The dermal fin-rays are opposed to the extremities of the branches, as in the Dipnoi. The pubic consists of a paired cartilage, to which tarsal pieces supporting the fin-rays are attached. The other existing Ganoid fishes have the spinous column entirely or almost entirely ossified ; these have been comprised under the common name Ilolostei. They form, however, three very distinct types ; several attempts have been made to co ordinate with them the fossil forms, but this task is beset with extreme difficulties, of which no satisfactory solution has as yet been advanced. The Polypteroidei have their spinous column formed by distinct osseous amphicoelous vertebrae, that FIG 38 _ Fore. limb of is, vertebrae with concave anterior Adpenser. and posterior surfaces. It is nearly diphycercal. The neural arches, though ossified, do not coalesce with the Pmx pmx- FIG. 39. Skull of Polypterds. (After Traquair.) I. Upper aspect of the primor dial cranium, with the membrane-bones removed. II. Lower aspect of the same. III. Side view, with the membrane-bones. IV. Lower aspect of the skull, part of the bones being removed on one side. The parts shaded with oblique lines are cartilage of the primordial skull. An, angular; ao, anteorbital ; Ar, articulary; B, basal; D, dentary ; E, ethmoid ;&amp;gt;, frontal; Ma, mastoid; Mp, metapterygoid ; MX, maxillary ; N, nasal ; 0, operculum ; Oc, occipital ; Pa, parietal ; PI, palatine ; Pmx, prsemaxillary ; po, postorbital ; Prf, prefrontal ; Pt, post-temporals ; Pif, postfrontal ; Ptr. pterygoid ; Q, quadrate ; S, suspenso- rium ; So, suboperculum ; Sp, sphenoid ; Spl, splenial ; St. supratemporals ; T, tympanic lamina ; Tu, turbinal ; v, vomer ; x x, small ossicles ; x x, spiraculars. centrum, and form one canal only for the myelon. There are no intermediate elements between the neural spines. [OSTEOLOGY. Interneurals are developed, but are simple, articulating with the dermoneurals. The abdominal vertebrae have parapophyses developed with epipleural spines. Only the caudal vertebrae have haemal spines, which, like the interhaemals, agree in every essential respect with the opposite neurals. Ribs are inserted, not on the para pophyses, but on the centra, immediately below the para pophyses. The skull of Polypterus (fig. 39) shows a great advance towards the Teleosteous type, the number of separable bones being greatly increased. They are arranged much in the same fashion as in Teleostei, but a great portion of the primordial cranium remains cartilaginous. The mem brane-bones which cover the upper and lower surfaces of the brain-case are so much developed as to cause the under lying cartilage to disappear, so that a large vacuity or fontanelle exists in the substance of the upper as well as of the lower cartilaginous wall. Of ossifications belonging to the primordial skull must be noticed the single occipital with a mastoid on each side. They are separated by persistent cartilage from the sphenoids and postfrontals ; the former, which are the largest ossification of the prim ordial cranium, enclose the anterior half of the brain cavity. Finally, the nasal portion contains a median ethmoid and a pair of praefrontal bones. Only a very small portion of the bones described are visible externally, nearly the whole of the primordial cranium being covered by the membrane-bones. Of these are seen on the upper surface a pair of parietals, frontals, &quot; nasals,&quot; and turbinals ; on the lower surface a large cross-shaped basal, anteriorly bordered on each side by a pterygoid, parallel to a palatine which forms a suture with the double vomer. The suspensorium has in front a metapterygoid and quadrate bone, and an operculum and suboperculum are attached to it behind. Praemaxillaries and maxillaries are now fully developed, but immovably attached to the skull. The lower jaw is ossified, and consists of an articulary, angular, dentary, and splenial. Of labial cartilages a rudi ment at the angle of the mouth has remained persistent. The side of the skull, in front of the operculum, is covered by a large irregularly-shaped bone (T) (correspond ing to the &quot; tympanic lamina &quot; of Ceratodus, fig. 35, &amp;lt;?), held by some to be the praeoperculum ; along its upper circumference lies a series of small ossicles, of which two may be distinguished as spiraculars, as they form a valve for the protection of the spiracular orifice of these fishes. An infraorbital ring is represented by a prasorbital and post- orbital only. Each hyoid consists of three pieces, none of which bear branchiostegals, the single median piece being osseous in front and cartilaginous behind. Four branchial arches are developed, the foremost consisting of three, the second and third of two, and the last of a single piece. There is no lower pharyngeal. Between the rami of the lower jaw the throat is protected by a pair of large osseous laminae (gular plates), which have been considered to represent the urohyal of osseous fishes. The scapulary arch is almost entirely formed by the well-developed membrane-bones, which in the ventral line are suturally united. The pectoral fin is supported by three bones, pro-, meso-, and meta-pterygium, of which the dilated middle one alone bears rays, and is excluded from the articulation with tho shoulder-girdle. The pubic consists of a paired bone, to which tarsal bones supporting the fin-rays are attached. In the Lepidosteoidei the vertebrae are completely ossified and opisthocoelous, having a convexity in front and a con cavity behind, as in some amphibians. Though the end of the body externally appears nearly diphycercal, the termination of the vertebral column is, in fact, distinctly