OCR Text |
Show 686 MR. F. G. PARSONS O N T H E [June 16, dorsal surface of the body of the axis, a line of cartilage was seen running transversely across just behind tbe level of the anterior articular facets ; this evidently corresponded to an inter-vertebral disc, and it is interesting to notice that, if the two parts of the bone had been separated here, the anterior would have corresponded very closely to the odontoid bone of Ornithorhynchus. In the older specimen of Petrogale with which I compared my own there was no indication of this separation, but in the disarticulated skeleton of a young Labillardiere's Wallaby the two parts of the axis were quite separate. Fig. 2. r I P ®J^ ! Axis of Petrogale. A, line of union of two parts. The following is the vertebral formula:-C. 7, T. 13, L. 6, S. 2, C. 24. The spines of the cervical vertebrae are short, and the neck is kept in an extended position by the very strong ligamenta subflava. The transverse process of the 6th cervical vertebra has a very prominent ventral tubercle, which forms quite an antero-posterior ridge. The thoracic spines are long, there being quite a sudden transition from the short 7th C. to the long 1st T. There are thirteen ribs, all of which except the first articulate with two vertebral centra, and all of which are supported by a transverse process. They are divided into 7 vertebro-sternal, 3 vertebrocostal, and 3 vertebral. O n the ventral side of the body of the 1st lumbar vertebra and just to the right of the mid-ventral line there is a single triangular bone fastened by its base to the centrum, while its apex projects ventrally ; it seems to be developed in the anterior common ligament, and is more closely attached to the posterior than to the anterior part of the vertebra. From its unilateral position I regard it as one of a pair of hypapophyses or intercentra which has worked forwards from the ventral side of the disc and which may possibly be homologous with the projection from the ventral side of the centrum of the 1st lumbar iii the Hare, although that process is median and has no separate centre of ossification. The lumbar transverse or costal processes have a sharp curve |