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Show 440 MR. J. W. HULKE ON THE SKELETAL [Nov. 20, limited material accessible to me shows, the plan of connection of the second rib in Allig. lucius approaches that in these Mesosuchia more closely than does that in Crocodilus and Gavialis. This I had not expected, and, in association with it, it should be remarked that in Alligator, as was noticed by D'Alton and Burmeister (45), the division of the vertebral end of the rib is better marked than it is in Crocodilus and Gavialis. The structural differences of the sacral vertebrae in the two Mesosuchian genera which form the subject of this paper have been already described; it remains to compare their plan with that occurring in the Eusuchia. In Steneosaurus, as also in Gavialis, Crocodilus, and Alligator, the anterior terminal surface of the first sacral vertebra, whilst principally composed of the true centrum, receives a considerable lateral accession from the root of the sacral rib, and a smaller complement from the neurapophysis. In Gavialis, Crocodilus, and Alligator, the posterior terminal surface of the second sacral vertebra also receives a large lateral accession from the root of the second rib, and in Alligator also a small complement from the neurapophysis; but in Steneosaurus and in Metriorhynchus (as illustrated by these remains) no part of the posterior surface of the second sacral vertebra is contributed by the rib. In Metriorhynchus the rib does not contribute any part of the anterior terminal surface of the first sacral vertebra ; and the accession to this surface from the neurapophysis is minute and inconstant. In Steneosaurus the accession to the posterior terminal surface from the neurapophysis is also a vanishing quantity. Thus in the plan of the sacral vertebrae there is a close agreement between that in Steneosaurus and in the Eusuchia mentioned which is not observed in Metriorhynchus. In Steneosaurus the plan of the sacral ribs also is very similar to that in Eusuchia, whilst in Metriorhynchus there are obvious differences, notably their greater length and slenderness, and their pronounced downward bend. The near resemblance in several skeletal details between these Mesosuchia and Alligator, to which attention has been called, and the very near ref-emblance of the dermal armour to that of J acar e is interesting as suggesting that the Alligatoridae may not have descended through the Gavials and Crocodiles; that these three genera may not represent successive phases of evolution, but rather three distinct lines of descent. This is not the generally accepted view, and it appears not to accord with the supposed first appearance of the three genera in time. List of Authors quoted. 1. CUVIER, G.-Reeherches sur les Ossemens Fossiles. 8vo ed. t. ix. p. 58. Paris, 1836. 2. O W E N , R.-Hist, of Brit. Foss. Rept. vol. iii. p. 129. London, 1849-84. 3. H U X L E Y , T.-Quart. Journ. Geol. Soc. Lond. vol. xxxi. p. 427. London, 1875. |
