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Show 212 ON THE STRUCTURE OF THE SKULL. suprascapular bones, as we II as, I·n some cases, of dorsal dermal bones. . f th mo~t stn.k i.n g mo dI' fi cat 'w ns of the p h ysi·- 9 But certain o e '"' . og·nom• y of osseous .H!! Sh- es are the result of the prolong.a tion of the . . f t f the orbit which may be effected In two very r~gwn In ron oF examp,le it is chiefly the elongation of the different ways. or J. • • • premaxi' llr e an d mandible whieh gives nse. to· thpe,· remz ar·k ab1le beak of the "sword-fish" (Xiphias) ; while, In ~stu artta, t 1e premaxi'1 1 ro and mandible rema.i n very short, bu·t are ft hhr ust ou1t to a great distance from the orbit, by the productiOn o t e nasa and vomerine regions, on the one hand, and of the bones of the suspensorium on the other. . . . . . 10. In such fishes as Syngnathus and F~stular~a, a hne JOin-ing the articular socket of the hyomandibular wi~h the condyle of the os quadratum, makes~ very a~ute angle with the base of the skull. In most fishes thts angle IS more or less aeut~; but in Polypterus, and still more in Murmna, it bec.oines a nght or an obtuse angle,' the corner of the gape being thus .thrown behind the eye, instead of being, as in most bony fishes, In front 0 f 't. We shall find a similar rotation of the distal end of the susIp ensorium to take place in the series of the Amph ~ ' b~. a, an d in the passage from the tadpole to the adult state of the highest of these animals. . 11. The connection of the _palato-quadrate areade With the hyomandibular and symplectic s~spensor ~aries, from the firm sutural union which is observed In the P1ke and most osseous fishes to a bond which is hardly closer than that which obtains in th~ Plagiostomes and Sturgeons, in Polypterus. -~n L epidosteus, except for the inter- and pre-operculum, the tie between the symplectic and the palato-quadrate bones would. be ver.y loose, the palato-quadrate arcade and the suspensor be1ng, as Jt were, naturally dissected from one another. In some Plectognathi and Siluroids, on the other hand, all these parts become firmly anchylosed together, and with the side walls of the cranium. 12. Finally, the multi.plic~tion o~ the bony constitue~ts o~ the maxilla and the mandible In Lep~dosteus-the conversiOn o the maxilla into a mere support for a tentacle in many Siluroids 'l'HE SKULLS OF AMPHIBIA. 213 -the absence of branchiostegal rays, and the presence of two "jugular " plates between the mandibular rami in Polypterus, must not be overlooked even in this brief enumeration of a few of the most salient modifications of the skulls of osseous fishes. THE SKULLS OF AMPHIBIA. In cranial structure, as in all the other more important features of their organization, the Amphibia are closely allied to Fishes, and widely separated from the abranchiate Vertebrata. As in Fishes, a single median membrane bone, or parasphenoid, is developed under the base of the skull, while no such median bone is found in the higher Vertebrata. Like Lepidosiren, the Amphibia have no ossified basi-occipital or supraoccipita], whereas a1l the abranchiate Vertebrata possess these bones. Again, like Lepidosiren and many other Fishes, the Amphibia have no true basi-sphenoid, developed in the cartilage of the basis cranii; while all the abranchiate Vertebrata have that bone well developed. The hyoidean apparatus is, in Amphibia, as in Fishes,* connected with a suspensorium common to it and the mandibular apparatus. In all the higher Vertebrata the hyoidean apparatus, if it is attached directly to the skull at all, is united therewith separately and distinctly. In all Amp~ib~a which have ossified ex-occipitals, a condyle is developed on each, for articulation with the first vertebra of the spinal column; and the basi-occipital, remaining unossified, takes no share in the formation of these condyles. In all the higher Vertebrata, on the other hand, the bony basi-occipital takes a greater or less share in the forn1ation of the occipital con.dyle, or condyles. The skull of A1nphibia resembles that of the Chimroroids and Lepidosiren, and .differs from that of Teleostean, Ganoid, and Plagiostome fishes, in the absence of any natural division between the palato-quadrate and suspensoria! cartilages. * According to Stannius, however, the hyoideau arch is attached directly and independently to the skull in many Rays. See that author's admirable " Handbuch clcr Anatomie dcr Wirbelthiere," Erster Buch, p. 46. |