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of much controversy. In a remarkable essay published in the first volume of the "Mémoires du Museum," Cuvier proposed to consider the upper dentigerous arch (h) as the homologue of the palatine and pterygoid bones of osseous fishes, the cartilages (i, k) as the premaxilla and maxilla. The suspensorium (g) he considered to be the homologue of the hyomandibular, symplectic, and metapterygoid. The lower dentigerous arch (Mn) was obviously the mandible.

On this latter point all anatomists are agreed; but, in his famous 66 Comparative Anatomy of the Myxinoid Fishes," Johannes Müller-guided, like Cuvier, by purely anatomical considerations, and by what I have elsewhere termed the method of gradation-proposed a totally different interpretation of the other parts. According to this view, i, k, and 7 are merely labial cartilages, and therefore do not represent the premaxilla and maxilla. Again, Cuvier had greatly relied upon. the absence of any parts on the inner side of h which could answer to palatine or pterygoid elements, in arguing that h itself represents them. But Müller adduced his own and Henle's observations to prove that in a great many Plagiostomes, particularly the Rays, such cartilages, situated on the inner side of the upper dentigerous arch, do occur, and thus arrived, by a line of argumentation precisely as legitimate as that of Cuvier, at the exactly opposite result,—that h represents the premaxilla and maxilla, and not the palatine or pterygoid.

The fact that these opposing views were entertained by men like Cuvier and Müller is evidence that each had much in its favour; but, in truth, neither was free from grave difficulties. Thus neither accounted for the articulation of the mandible with the upper dentigerous arch,-a relation into which the mandible never enters either with the palatine, or with the maxilla, in the vertebrate series; and as Müller himself is forced to admit that some of the cartilages on the inner side of the upper dentigerous arch are accessory, why should not all be so?

This is just one of those cases in which the study of development manifests its full importance, and decides, at once, problems which, without it, might be the subjects of interminable discus

sion. A comparison of the skull of the monk fish with that of the embryonic osseous fish (Fig. 72, C) seems to me to demonstrate beyond question, that the upper dentigerous arch (h) corresponds with the palato-quadrate cartilage of the embryo,* and that the suspensorium (g) equally corresponds with the hyomandibular and symplectic cartilage. But in this case Cuvier's view of the upper dentigerous arch must be regarded as a singularly near approximation to the truth, for it certainly answers to the palatine and pterygoid; though, in addition, it contains the representatives of the quadrate and metapterygoid bones of the osseous fish. And his opinion regarding the nature of the suspensorium was still nearer to what I believe to be right. On the other hand, I think it very probable, though not certain, that, as Müller supposed, the cartilages (i, k, l) are merely labial, and that these fishes have no representatives of the premaxilla and maxilla. But the so-called palatine and pterygoid cartilages of Müller, if the view I take is correct, are as much accessory parts as the spiracular cartilages, and, like them, have no representatives in osseous fishes.

* Rathke arrived at this conclusion also, on developmental grounds, in 1839. See his "Vierter Bericht," quoted in the last Lecture of this work.

202

LECTURE XI.

ON THE STRUCTURE OF THE SKULL.

THE SKULLS OF FISHES AND AMPHIBIA.

C. The cranium, consisting chiefly of cartilage and without cartilage bones, but with superadded membrane bones.

The skulls of the chondrosteous Ganoids, the Sturgeons, and Spatulariæ exemplify this type of structure, which forms a most interesting transitional link between the skull of Plagiostomes and the skull of ordinary osseous fishes.

Spatularia has a completely cartilaginous skull, produced in front into a great beak, flattened from above downwards. The cartilaginous representatives of, at fewest, seven of the anterior Fig. 81.

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Fig. 81. Side view of the skull of Spatularia with the anterior (asc) and posterior (psc) vertical semicircular canals exposed.-Au, the auditory chamber; Or, the orbit with the eye; N, the nasal sac; Hy, the hyoidean apparatus; Br, the representatives of the branchiostegal rays; Op, the operculum; Mn, the mandible.

vertebræ of the spinal column coalesce into one mass with one another and with the skull. The notochord, extremely large in the spinal column, rapidly diminishes in size as it enters the skull, and, becoming a mere thread, terminates behind the pituitary fossa. The auditory organs are contained in large postero-lateral projections of the cranial mass, with the outer sides of which the suspensoria are connected. The base of the skull is protected by a long parasphenoid, which extends back under the anterior part of the spinal column; in the dorsal region it presents an anterior and a posterior pair of perichondrial ossifications, separated by oblong laminæ from lateral bony plates of the same character, but the homology of these bones with those in the roof of the Teleostean skull is not, to my mind, satisfactorily made out.*

The suspensorial apparatus of Spatularia consists of a single bone (4), compressed from above downwards superiorly, and from side to side inferiorly, with a superior and an inferior cartilaginous epiphysis; to the lower cartilaginous epiphysis the operculum (Op) is attached, and a short thick prismatic cartilage (B) is united by ligament with, and can play freely upon, its anterior and inferior angle. Posteriorly the lower end of this cartilage (B) is connected by ligament with the hyoidean arch (Hy), which consists of two portions on each side; a small upper piece, with which the flat bone (Br), representing a branchiostegal ray, is connected; and a long lower ramus, the middle third of which is bony, while the two ends are cartilaginous.

Anteriorly, the lower end of the inferior suspensorial cartilage (B) is united by ligaments to two cartilaginous semi-arches (D and Mn), of which the upper (D) is articulated by a transversely convex head with a concavity of the lower (Mn). The upper semi-arch is ligamentously united to its fellow in the middle line, and is suspended by ligamentous fibres to the under part of the prefrontal region of the skull. A long flat bone (E), the hinder end of which is cut off in the specimen figured, lies on the outer side of the cartilage (D), and extends to the middle line. A second long flat bone is closely applied to the inner

* See "Spatulariarum Anatomiam descripsit Tabulaque illustravit Albertus Wagner." Berolini, 1848.

surface of the cartilage and follows its curves, from its hinder to its anterior extremity, overlapping and folding over the upper edge of the anterior three-fifths of the cartilage. Between the hinder part of E, here cut away, and D, is a space occupied by the levator muscle of the lower jaw.

The mandibular cartilage extends to the symphysis, and is coated externally, and partially embraced by, the flat bone (Mn), the greater part of the upper edge of which bears teeth.

On comparing these parts with those of the corresponding apparatus in the embryonic fish (Fig. 72), it becomes clear that the pieces A and B answer to the hyomandibular and symplectic, taken together. Indeed, at first sight, A, supporting as it does the operculum, seems to answer to the hyomandibular, and B to the symplectic itself; but then it may be suggested that the hyoidean apparatus is attached at the distal end of B, and not between it and A, as it would be if the two corresponded, respectively, to the hyomandibular and symplectic.

The cartilage D obviously answers to the palato-quadrate arch, and that of the lower jaw to Meckel's cartilage. The fact that a levator muscle of the lower jaw passes between E and D seems to prove the former to correspond with a maxilla; in which case the internal bone would be a sort of palatopterygoid, similar to that we shall meet with in Lepidosiren.

The skull of the Sturgeon (Accipenser), like that of Spatularia, is greatly enlarged, posteriorly, by the coalescence with it, and with one another, of six or seven of the anterior vertebræ. In front, it is prolonged into a triangular snout or beak (c, Fig. 82; a, Fig. 83), the wide base of which is formed by the antorbital, or prefrontal, prominences which separate the olfactory chambers from the orbits. Behind the latter are the two great projections (c, Fig. 83) which contain the auditory organs; and behind these again, and separated from them by a deep lateral fossa, are two wing-like processes (b, Fig. 82), which are directed outwards and obliquely backwards, and proceed, not from the walls of the cranium proper, but from those of the spinal column, where it joins the skull. At this point there is, in the craniospinal cartilage of both the Sturgeon and the Spatularia, a great dilatation of the neural canal, which is closed above only by a

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