Page:Encyclopædia Britannica, Ninth Edition, v. 12.djvu/665

647 OSTEOLOGY.] ICHTHYOLOGY 647 FIG. 35. Palatal view of skull of Ceratodus. lary and intermaxillary elements are not developed, but are perhaps represented in Ceratodus by some inconstant rudi mentary labial cartilages situated behind the posterior nasal opening. Facial cartilages and an infraorbital ring are de veloped, at least in Ceratodus. The presence of a pair of small teeth in front indicates the vomerine portion (v) which remains cartilage, whilst the posterior pair of teeth are im planted in a pterygo- palatine ossification (/), which sometimes is paired, sometimes con tinuous. The base of the skull is constantly covered by a large basal ossification (o). The hyoid is well developed, sometimes reduced to a pair of ceratohyals, sometimes with a basi- hyal and glossohyal. The skeleton of the branchial apparatus ap proaches the Teleosteous type, less so in Lepido- nren than in Ceratodus, in which five branchial arches are developed, but with the lateral and mesial pieces reduced in number. A large oper- culum, and a smaller sub- or inter-operculum are present. The scapular arch consists of a single median transverse cartilage, and a pair of lateral cartilages which bear the articular condyle for the pectoral limb. The latter cartilages form the base of a large membrane-bone, and the whole arch is suspended from the skull by means of an osseous supraclavicle. The fore-limb of the Dipnoi (fig. 36) greatly differs ex ternally from the pectoral fin of other Ganoid fishes. It is covered with small scales along the middle, from the root to its extremity, and sur rounded by a rayed fringe similar to the vertical fin (crossopterygian type of fin). A muscle split into numerous fascicles extends all the length of the fin, which is flexible in every part and in every direction. The cartilaginous framework sup porting it is joined to the scapular arch by an oblong cartilage, followed by a broad basal cartilage (a), gene rally single, but sometimes showing traces of a triple division. Along the middle of the fin runs a jointed axis (b], the joints gradually be coming smaller and thinner towards the extremity ; each joint bears on each side a three-, two-, or one- jointed branch (c, d). This axial arrangement of the pectoral skeleton, which evidently represents one of its first and lowest conditions, has been termed the archipterygium by Gegenbaur. It is found in Ceratodus and other genera, but in Lcpidosiren the jointed axis only has been preserved, while rudimentary rays are added in Protopterus. The pubic consists of a single flattened subquadrangular cartilage, produced into a long single anterior process. Posteriorly it terminates on each side in a condyle, to which the basal cartilage of the ventral paddle is joined. FIG. 3fi. Fore-limb of Ceratvdus. The endoskeleton of the paddle is almost identical with that of the pectoral. The Ganoid fishes with persistent notochord, but with a hyostylic skull (that is, a skull with a separate sus- pensorium), consist of the suborder Chondrostei, of which the existing representatives are the sturgeons (Acipenser, ScapJiirhynclms, Polyodon), and the extinct the Chon- drosteidce, Paloeoniscidce, and (according to Traquair) Platy- somidce. Their spinal column does not differ essentially from that of the Dipnoi. Segmentation is represented only so far as the neural and haemal elements are concerned. All are eminently heterocercal. Ribs are present in most, but are replaced by ligaments in Polyodon. The primordial cranium of the sturgeons consists of per sistent cartilage without ossifications in its substance, but superficial bones are still more developed and specialized than in the Dipnoi ; so it is, at least, in the true sturgeons, but less so in Polyodon (fig. 37). The upper and lateral FIG. 37. Skull of Polyodon. (After Traquair.) n, nasal cavity ; s&amp;lt;7, squamosal ; mh y hyomandibular ; sy, symplectic ; pa, paluto-pterygoid ; m, Meckelian carti lage; mx, maxillary ; d, dentary; h, hyoid; op, operculum ; br, brunchiostegal ; s.d, supraclavicular ; p.cl, postcluvicular ; cl, clavicle ; i.cl, infrucluvicular. parts of the skull are covered by well-developed membrane- bones. The lower surface of the skull is covered by an ex tremely large basal bone, which extends from the vomerine region on to the anterior part of the spinal column. The nasal excavation in the skull is rather lateral than inferior. The ethmoidal region is generally much produced, forming the base of the long projecting snout. The suspensorium is movably attached to the side of the skull, and consists of two pieces, a hyomandibular and a symplectic, which now appears for the first time as a separate piece, and to which the hyoid is. attached. The palato maxillary apparatus is more complex than in the sharks and Dipnoi ; a palato- pterygoid consists of two mesially-connected rami in Polyodon, and of a complex cartilaginous disk in Acipenser, being articulated in both to the Meckelian cartilage. In addition, the sturgeons possess one or two pairs of osseous rods, which, in Polyodon at least, represent the maxillary, and therefore must be the representatives of the labial car tilages of the sharks. The Meckelian cartilage is more or less covered by tegumentary bones. In the gill-cover, besides the operculum, a sub- and inter-operculum may be distinguished in Acipenser. The hyoid consists of three pieces, of which the posterior bears a broad branchiostegal in Polyodon. In the scapulary arch the primordial cartilaginous elements scarcely differ from those of the Dipnoi. The membrane-bones are much expanded, and show a continu ous series suspended from the skull. Their division in the median ventral line is complete. The pectoral is supported by a cartilaginous framework (fig. 38) similar to that of Ceratodus, but much more shortened and reduced in its periphery, the branches being absent altogether on one side of the axis. This modification of the fin is analogous to the heterocercal condition of the end of the spinous column. To the inner corner of a basal cartilage (a) a short axis (b) is joined, which on its outer side bears a