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Show 206 ON THE STRUOTUHE OF THE SKULL. the prefrontals, G, G to the post-fronta~, B, B to the. epiotics, and A to the supra-occipital of the Pike. But every one. of these is a membrane bone, and not, as are the corresponding elements of the Pike's skull, a cartilage bone .. These bones are therefore, strictly spealnng, the analogues, and not the homologues, of the bones to w~ic~ t~ey appear to answer in the Pike, though hitherto no distinction has been drawn between the two. K and I 1 are bones which do not properly belong to tho skull, but ,~hich, as happens among ma~y Siluroid Teleost.ei, are anchylosed with the cranium. K 1s the most antenor of the median dermal scutes, and I, 1 are the supra-scapular bones. The letters L, L indicate the scapular bones move-ably united with these last. . . The suspensorium of the Sturgeon consists of a .large, In:e-gularly- prismatic body, composed of a bony central pie?e (f, FJ ~. 83) with two cartilaginous epiphyses, the _lower of ~hwh (g) 1s much the longer, and is connected by hgament With another cylindrical cartilage (h), while the upper articulates with the outer and under part of the auditory capsule. Rather above the letter h, the lower cartilage gives attachment to a cartilaginous nodule with which the principal piece of the hyoidean arch is connected. At its distal end the cylindrical cartilage (h) is united by ligaments to the two apparatuses !1; and Mn. Of these, k may be described as a rhomboid, composed partly of cartilage and partly of bone, and so bent as to assume a transversely arched form. Its outer angles present convex articular facets to concav~ ties on the lower arch (Mn), which last., cmnposed of a single bony ramus on each side, is undoubtedly the mandible. Fig. 83. Fig. d3.-Side view of the cartilaginous cranium of Accipenser (after Mtiller) .-a, rostrum; b, nasal chamber; Or, orbit; c, region of the auditory organ; f, g, h, suspensorium; ll, maxillo-palatine apparatus ; Mn, maudilJlc. t THE SKULLS OF FISHES AND AMPHIBIA. 207 The cartilagi~ous basis of k is strengthened by eight bones, four on each side. Of these, two lie altogether external to the car~ilage, and leave between themselves and it an interspace, in whwh the levator muscle of the lower jaw lies. The other pair ~onsist, firstly, of a large bone, which lies, for the n1ost part, Internal and inferior to the cartilaae and extends from the inner side of the articular process for th~ lower jaw, upwards ~nd inwa:ds, to meet its fellow, posteriorly; forwards, to articulate w1th the anterior of the external bones. And, secondly, of a small bone fitted on to the anterior and external edge of this, and to the inferior surface of the anterior external .bone. The whole apparatus, k, is very loosely connected With the skull, so that it is capable of beino- protracted and retracted with great freedom. 0 The general relations of this singular mechanisrn to the manducator! organs of ordinary Teleostean fishes appears to be rendered evident by the same method as that which has been employed to demonstrate the nature of the jaws of the Plagiostomes. The osseo-cartilaginous structure, !1;, answers to the palato-quadrat~ arch of the Sharks and Rays, or to the palataquadrate. cartilage of the embryonic fish ,· and j,, g , h co rre-spond .with the undivide.d suspensorium of the Sharks and Rays, and With the hyomand1bular and symplectic cartilages of the emb~yo Teleostean. Furthermore, on cmnparing k with the maxillary apparatus of Spatularia, the cartilaginous basis appears to ~nswer to th~ cartilages (D, D) of that fish joined together; while the anteriOr outer bone in the Sturgeon is the equivalent ?f the bone E, and may be regarded as a maxilla. The two ~nt~r,na~ bones correspond with the inner bone of the Spatu-an~ s Jaw. The Sturgeon, however, more nearly approacheR ordinary ~sh.es in the development of an anterior or palatin~ element, distinct from the posterior or pterygoid element. As for the small external bone, which passes obliquely from the end of the maxi~la to the outer surface of the cartilage, it is possibly a quadrato-Jugal. D. The cranium consisting of car~ilage to a greater or less extent, but with cartilage bones as well as membrane bones.- |