WEALDEN CROCODILES. 419 



at one part by pressure. The medullary cavity of the bone is well marked, and bears 

 to the compact wall the proportion shown in fig. 4, PL 38. 



From the foregoing scanty data relative to the Pelorosaurus, and on the supposi- 

 tion of the long-bone being, as I believe it to be, a humerus, it may be inferred 

 that there coexisted at the Wealden period, with the Iguanodon, Megalosaiirus, and 

 Hylceosaurtts, a reptile of more Crocodilian affinities, and of a bulk at least equalling 

 that of the largest of these Dinosauria. 



In the characters of the best-preserved vertebrae— those, viz., from the base 

 of the tail, — the Pelorosaurus most resembles the Iguanodon ; and the differences 

 here observable may not be of more than specific importance : the Crocodilian 

 character of the humerus points, however, to a generic distinction. 



From the Cetiosaurus the Pelorosaurus is more obviously and decidedly distinct, 

 by vertebral characters, which, in regard to the latter genus, have now been, for 

 the first time, pointed out. 



The genera of Saurian reptiles, hitherto determined, from the Wealden strata, have 

 been founded on vertebral characters. With these, in regard to two of the genera, 

 viz., Iguanodon and Megalosaiirus, corresponding generic distinctions have been 

 yielded by the teeth. The same may be affirmed, with a high degree of probability, 

 but not as yet with certainty, in respect of the Hylceosaurus. There is a fourth 

 form of tooth, generically distinct from the foregoing, applicable in respect of 

 size to either Cetiosaurus or Pelorosaurus. 



Not any of the foregoing genera have been founded on the structure of the 

 limb-bones ; for, indeed, sucli structure is not generic. Some of these bones, 

 for example, may be hollow, and others solid in the same limb of the same reptile. 

 The femur of the Cetiosaurus might have a small medullary cavity, whilst the tibia, 

 the fibula, and the metatarsal were cancellous in the centre. The generic distinction 

 of this huge reptile was originally, and in every subsequent descriptions of its 

 specifically differing remains, founded upon A^ertebral characters. The names of 

 the species bear reference to the proportions and minor modifications of essentially 

 Cetiosaurian vertebrte. If, therefore, the long-bone — most probably humerus — • 

 above described, should belong to the same species as the Cetiosaurus hrevis, and 

 not to the very distinct species established in the vertebrae marked Pelorosaurus 

 by Dr. Mantell in his last collection, the medullary cavity of the Crocodilian bone 

 would be no sufficient ground for suppressing the genus. 



Neither, supposing the appearance of the cancellous centre of the equally long 

 limb-bone of the great Saurian from the Bradford Clay at Enslow Bridge, Oxford- 

 shire, to be due to compression, obliterating the medullary cavity, would that afford 

 just and satisfactory ground for determining the genus of reptile to which the 

 crushed bone belonged. The tibia of the correspondingly large reptile from the same 

 formation and locality, originally deemed to be Cetacean, is, indeed, solid ; but 



2 n 



