440 MR. J. W. HXJLKE ON THE SKELETAL [NoV. 20, 



limited material accessible to me shows, the plan of connection of 

 the second rib in Alliff. 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 vertebrse 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. la Steneosaurus, as also in Gavialis, Crocodilus, 

 and Alligator, the anterior terminal surface of the 6rst sacral 

 vertebra, whilst principally composed of the true centrum, receives a 

 considerable lateral accession from the root of the sacral rib, and a 

 smaller complenient from the neurapophysis. In Gavialis, Cro- 

 codilus, 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 Metriorhjnchus (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 agree- 

 ment 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 resemblance of the dermal armour to that oi Jacare is 

 interesting as suggesting that the Alligatoridse 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. — Recherches sur les Ossemens Fossiles. 8vo ed. 



t. ix. p. 58. Paris, 1836. 



2. Owen, R. — Hist, of Brit. Foss. Rept. vol. iii. p. 129. London, 



1849-S4. 



3. Huxley, T. — Quart. Journ. Geol. Soc. Lond. vol. xxxi. p. 42". 



London, 18/5. 



