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Show 1896.] MR. G. A. BOULENGER ON SCHLEGEL's GAVIAL. 631 hypapophyses on the cervical and anterior thoracic vertebrae are less developed than in the other recent Crocodilians, and are not directed forwards ; they are not developed beyond the eleventh vertebra (twelth or thirteenth in the others). The chevron-bones are all open dorsally. The first pair of ribs are inserted on the sides of the proatlanto-atlantic hypapophysis, or lower part of the atlas-ring, and separated from each other at the base by a wide interspace. The second rib differs from that of all Crocodilians I have hitherto examined (including the Gavial, of which I have examined the bone on a young specimen in spirit, and also the atlas and axis preserved in the M u s e u m of the Royal College of Surgeons,-the Gavial-skeleton still being a desideratum in the British Museum Collection), Osteolaemus excepted ; it is attached to the centrum of the atlas (odontoid bone), near its suture with the axis, by the capitulum only, the tuberculum being merely indicated by a small upward process at a distance from the base of the bone, and without any connection with the vertebrae. It is well known that in Ichthyosaurus the atlas bears a forked rib, same as the axis and tbe other vertebrae behind it. It seems that one Crocodilian at least presents an approximating feature. The late M r . Hulke has first pointed out in Metriorhynchus (P. Z. S. 1888, p. 419) the presence on the " lateral pieces " (neurapophyses) of the atlas of a tubercle situated in the level of the diapophysis on the epistropheus, and he concludes that this tubercle should rank as an upper atlantal transverse process or diapophysis. I have been able to verify the correctness of this statement on several well-preserved atlases of Metriorhynchus, still undescribed, from the Leeds Collection, which m y colleague M r . Andrews has kindly shown m e in the Geological Department of the British Museum ; and I quite agree with Hulke that " the position of this little process in serial line with the upper transverse processes of the other cervical vertebrae speaks distinctly in favour of its diapo-physial character." W e are, in consequence, justified in assuming that, although, as w e know from one specimen, the first rib is not forked, it must have been connected with the diapophysis by ligament, its head being attached to the side of the hypapophysis (" basilar piece ") of the atlas, or rather between the latter and the centrum (odontoid bone); and such a condition may be regarded as the most primitive known among Crocodilians, and as one from which, as Hulke has shown, the abnormal position of the first rib of recent forms m a y be derived and explained. The second rib in Metriorhynchus was attached by its capitulum to the anterior border of the lower surface of the centrum of the axis, or between the latter and the centrum of the atlas, and by its tuberculum to a process (diapophysis) of the neurapophysis of the axis. As regards recent Crocodilians, the information to be derived from books appears contradictory, principally from the fact that the various authors have dealt with different genera, and have in some cases generalized their observations to the whole group. |
