| OCR Text |
Show 188 PROF. T. H. HUXLEY ON MENOBRANCHUS. [Mar. 17, fig. I), presents the form of a pentagon, with a deeply excavated base, and with the angle opposite the base truncated. This truncated angle corresponds with the ends of the premaxillary bones; the lateral angles are occupied by the extremities of the suspensoria, or peduncles to which the mandible is attached; the posterior angles answer to the epiotic processes of the skull of an osseous fish. As these project beyond the level of the occipital foramen, they give rise to the excavated contour of the base of the pentagon. The occipital condyles lie one on each side of and below the occipital foramen; and their slightly convex free surfaces look inwards and backwards. In a side view (Plate XXIX. fig. 1), the skull is seen to be much flattened from above downwards. The suspensorium is inclined downwards and forwards at an acute angle with the cranio-facial axis. The ramus of the mandible is proportionally stout and thick, and the hyobrancbial apparatus, though almost wholly cartilaginous, is massive and large relatively to the skull. The skull consists of a cartilaginous framework, in and upon which certain ossifications have been developed. The former is what is commonly termed the "primordial cranium;" but, as it is preceded by a membranous structure, it would be better termed the " chondrocranium," while the bony skull may be called the "osteo-cranium." The osteocranium consists of the following bones :- 1. The exoccipitals (E.O).-These lie on each side of the occipital foramen, and bear the condyles. They do not come into contact either above or below; but the small space left between them is covered by the parietal bones in the former region, and by the parasphenoid in the latter. 2. The epiotics (Ep.O).-I term these interesting ossifications " epiotic " for brevity's sake. In reality they represent not only the epiotic but the opisthotic ossifications of other Vertebrata. They are conical caps of bone, which are separated by narrow cartilaginous interspaces from the pro-otics (Pr.O) in front, but closely unite with the exoccipitals behind and below. As usual, they shelter the posterior part, of the organ of hearing; and the fenestra ovalis, with its stapes (St), is situated in the unossified interspace between the anterior and inferior (or opisthotic) part of the bone and the pro-otic. 3. The pro-otic ossifications (Pr.O) occupy their ordinary place in the front part of the capsule of the organ of hearing, and are covered above by the parietals (Pa.), and externally by the squamosals (Sq.). 4. The parasphenoid (P.Sph) is a very wide and thin bone, which extends from the lower margin of the occipital foramen, posteriorly, to a point beyond the middle of the length of the vomers, anteriorly. It underlies the exoccipitals, the epiotics, and the pro-eten Segmente der eigentlicben Schedelcapsel nicht ossificirt," and that they have an abortive cartilaginous antorbital process. H e also mentions that the ligament which extends from the pratmaxilla to the suspensorium contains particles of cartilage fein^esprengte Knorpel), I have not noticed these in m y specimen. |
