| OCR Text |
Show 190 PROF. T. H. HUXLEY ON MENOBRANCHUS. [Mar. 17, which I examined, there was no maxilla whatever, mere fibrous tissue connecting the end of the preemaxilla with the outer extremity of the antorbital process, and bounding the posterior nasal aperture externally. But in the skeleton of Menobranchus prepared by Hyrtl, now in the Museum of the Royal College of Surgeons, there is, on the right side, a minute bone bearing three teeth, which seems to be a rudimentary maxilla. 11. The squamosals (Sq) are long slender bones, which extend from near the extremity of the epiotic processes to the articular surface for the mandible on the extremity of the suspensorium. The whole bone is shaped somewhat like a boomerang-the half which lies against the outer side of the suspensorium being bent at an obtuse angle to the half which is connected with the pro-otic, parietal, and epiotic bones, and which runs parallel with the axis of the skull. Where the cranial and suspensorial portions of the bone meet, there is given off, from its posterior margin, a short osseous process, which is directed towards the stapes, covering over the ligamentous fibres which connect the stapes with the suspensorium. 12. The dentary (D) occupies the whole length of the mandible, rising up on its outer side into a high plate, curving inwards to the symphysis in the middle line, and extending as a shelf, grooved above, beneath Meckel's cartilage, which is received into the groove. Fourteen conical teeth are borne by this bone, and are ankylosed with it. 13. The splenial (Spl, Plate X X X I . fig. 7) lies on the inner side of Meckel's cartilage, and bears six teeth. 14. The only other ossified member of the cranial, or facial, series present in Menobranchus is the second basibrancbial (Eb2, Plate X X X . fig. 2), a styliform bone, broader in front than behind, which lies in the middle line, and articulates, in front, with the two anterior cerato-branchials (Cb1). The chondrocranium, in and upon which the bones now enumerated and described are developed, is a structure of an exceedingly remarkable character. The whole extent of the chondrocranium was ascertained by decalcifying the skull, macerating it afterwards in glycerine containing caustic potash, and then, partly, viewing it as a transparent object, and, partly, examining sections. In the side walls of the depressed cranial cavity, between the parasphenoid and the second process of the parietal bone (Pa2), there lies a rod of cartilage, which, at the anterior boundary of the orbit, is connected with the tapering antorbital process (A.O). It then bends inwards (on the inner side of the olfactory sac) ; and, meeting its fellow in the middle line, the two proceed, parallel with one another, to the end of the snout, their free extremities being embraced by the prsemaxillae. The antorbital process is separated from the chief mass of this rod, which corresponds with one of the trabecula of the ordinary embryonic vertebrate cranium, by a line of fibrous tissue. In the internasal region, on the other hand, the anterior ends of the cartilaginous rods (which answer to the cornua of the trabecules) become fused together. |
