| OCR Text |
Show 38 PROF. T. H. HUXLEY ON CERATODUS FORSTERI. [Jan. 4, one specimen, the hinder end of each inner lateral bone is completed by a distinct ossification (C). There is also a separate ossification (E) on the left side, on what corresponds with the front part of the outer lateral bone on the right side. Doubtless these bones are subject to much individual variation. The fibrous band which extends, below the eye, between the antorbital process and the ventral end of the suspensorium contains three suborbital bones. The anterior of these, trihedral, is connected by its broad base with the antorbital process ; the middle bone is slender and elongated ; the posterior is broad, flattened from side to side, and its hinder edge is fixed by ligamentous fibres to the outer face of the suspensorium and of the squamosal. The basal bone of Dr. Gunther is the parasphenoid. It extends backwards, beyond the limits of the proper cranium, into the region of the vertebral column, to a point just beyond the attachment of the third pair of ribs. But there is at least one vertebra in front of that which bears the first pair of ribs. In Ceratodus, as in the Sturgeon and other Ganoids, several anterior vertebrae have coalesced with one another and with the skull; or, probably, it would be more accurate to say that the investing mass of the notochord has not become differentiated into vertebrae for this extent. Nevertheless the posterior boundary of the skull can be strictly defined by the interspace between the exit of the pneumogastric and that of the next following, or first spinal, nerve. It is to the outer surface of this interspace that the anterior edge of the "suprascapular" element of the pectoral arch is fixed by strong ligamentous fibres (fig. 2). Just in front of the boundary between the skull and the vertebral column, and therefore in the side walls of the former, there lies, deep in the substance of the cartilage, a hollow cone of bone (E.O) lt is wider above and externally than below and internally, where its end lies above the notochord. This appears to be an exoccipital ossification, such as is to be found in greater state of development in Lepidosiren, Polypterus, and Menobranchus. The skull of Ceratodus is, as might be expected (and as Dr. Gunther has pointed out), extremely similar to that of Lepidosiren. In fact, beyond differences in the proportions of its various parts, the more extensive fenestration of the roof of the olfactory capsules in Lepidosiren, and the absence, so far as m y investigations have yet gone, of the hyomandibular cartilage in the latter genus, the cartilaginous elements of the skull are the same in the two cases. As to the superadded bones, the parasphenoid, the rudimentary vomers, and the pterygopalatine plates correspond in the two genera. The exoccipitals are much larger in Lepidosiren. The descending process or praeopercular part of the squamosal is best developed in Lepidosiren, whilst its dorsal part (proper squamosal) is larger in Ceratodus. In both, there are two opercular bones, an operculum and an interoperculum ; and in Lepidosiren, as in Ceratodus, there are cartilaginous plates attached to the inner faces of these bones. |
