Weal den of Brighstone Bay. 447 



of skeleton can no longer be used as characters separating the two 

 species. Thus it becomes a moot point whether the differences to be 

 found in the skeletons of the two forms are not after all sexual, for 

 the type is too marked to be a case of individual peculiarities and the 

 divergent characters do not seem sufficiently great to be of specific 

 value. 



Mr. G. A. Eoulenger^ thought that the presence of six vertebrae 

 in the sacrum of /. bernissariensis and Hve in that of Lmantelli proved 

 the former a different species from the latter, but M. P. J. Van 

 Beneden ^ held that the variation in the number was only due to 

 individual differences, and that the number varied in Dinosaurs from 

 four to six. Professor L. Dollo ^ later strongly supported the 

 conclusions of Mr. G. A. Boulenger, considering that the obliteration 

 of the cranial sutures and of the neuro-central suture of the vertebrae 

 and the presence of a foramen in the coracoid of the small form, 

 which had obviously communicated during youth with the exterior^ 

 excluded the idea of making them young individuals, and that there- 

 fore the large form could not thus represent the adult state of the 

 small. Neither did he think that the divergences could be explained 

 by individual variations, and although the smaller size, greater 

 gracefulness, and the reduction of the thumb might be interpreted 

 as sexual variations, the different form of the pelvis and the number 

 of the sacral vertebrae would not admit of this interpretation. 

 Therefore he concluded that individual variations and sexual characters 

 were insufficient to explain the two forms. 



With regard to the specimen of the sacrum in the British Museum, 

 1^0. 37685, Hulke " many years ago doubted its belonging to /. mantelU, 

 and after an examination of it 1 am convinced that he is correct. 



The presence of only five sacral vertebrae in the form known as 

 /. mantelli may possibly be connected with the situation of the median 

 trochanter in the middle third of the shaft of the femur. Such 

 a position by curtailing the length of the caudo-femoral muscle would 

 not allow so great a stride as in /. hernissartensis, where it is placed 

 in the lower third. The swaying of the ponderous tail would be less, 

 and consequently there would be a decreased strain on the sacral 

 vertebrae. In /. beniissartefins the greater stride and probably 

 increased activity necessitated more rigidity in the sacral region, the 

 result being six instead of five sacral vertebrae in that form. 

 Moreover, I suggest that /. mantelli was the female and 

 /. hernissartensis the male of the same species, and that this Dinosaur 

 was viviparous. There is evidence that some were, for Marsh ^ declares 

 that there is an embrj'o within the ribs of the type-specimen of 

 Compsognathus, and it is a well-known fact that among the 

 Ichthyopterygia, Ichthyosaurus was. If this were the case in Iguanodon, 

 the differences found in the sacrum, pelvis, and femur need no further 

 explanation. The expanded preacetabular extremity of the pubic 

 bone into a large circular plate in /. mantelli is strikingly different 

 from the spatulate form of /. hernissartensis. Is it not possible that 



■^ Yide stipra. 



■ J. W. Hulke, Quart. Journ. Geol. See, vol. xxxvi, p. 447, 1880. 



^ Marsh, Amer. Journ. Sci., ser. Ill, vol. xxii, p. 340. 



