WEALDEN DINOSAURS. 371 



parts of the bodies of the vertebrae are towards the observer, and their spines imbedded 

 in the matrix. The coracoids (52) and scapulae (51) are placed, as might be expected 

 in a skeleton little disturbed and lying on its back, with their under surfaces towards 

 the observer, and covering, like a buckler, a portion of the vertebras and ribs. In 

 this position we might look for a portion of the apparatus of the sternal or abdominal 

 ribs, in the hope of discerning the modifications of these variable parts which might 

 characterise a genus differing in many peculiarities from other known Saurians. 

 Now it is with the apparatus of abdominal I'ibs, which present such a diversity of 

 characters in other Saurians, that it may be useful to compare the long flattened bones 

 in question, as well as with the supporting bones of a dorsal crest, in the event of a 

 future discovery of a skeleton or portion of skeleton of the Hylseosaurus including 

 these bones. The objection to their being abdominal ribs, which may be founded 

 on their great relative breadth as compared with those ribs in other Saurians, and 

 especially with the vertebral ribs of the Hylseosaurus itself, deserves due considera- 

 tion ; but the same objection applies to the bones in question as compared with the 

 superadded spines in the Lizard with a dorsal fringe, or with the spines of the 

 vertebrae themselves in the Hylaeosaurus. For the dorsal dermal spines in the Cyclura 

 correspond in number with the spines of the vertebrae which support them, while the 

 base of each of the hypothetical dermal spines of the Hylaeosaurus extends over more 

 than two vertebras. 



In the Monotrematous quadrupeds {OrnithorJiyncIms and Echicha) the abdominal ribs 

 are as much broader than the vertebral ribs as they would be in the Hylaeosaurus, on 

 the costal hypothesis of the detached bony plates here suggested ; and, after the close 

 repetition in the Ichthyosaurus, of another of the remarkable deviations in those 

 aberrant Mammals from the osteological type of their class, viz., in the structure of their 

 sternal and scapular arch, the reappearance of the monotrematous modification of the 

 sternal ribs in the present extinct reptile would not be surprising. The want of 

 symmetry and the difference of size and form, above alluded to, in the four succeeding 

 spine-shaped plates, agree better with the costal than the spinous hypothesis. 



Whether the bones in question be dorsal spines or abdominal ribs, they have 

 evidently been displaced from their natural position in the partial disarticulation of the 

 entire skeleton (PI. 3.5) prior to its immersion in the mud that has been subsequently 

 hardened around it ; but the degree of displacement has not been greater in the one 

 case than in the other. 



In offering, with due diffidence, a choice of opinions respecting the nature of these 

 singular bones, I have been actuated solely with the view of accelerating the acquisition 

 of the true one ; which, it is obvious, will be more likely to be attained by the choice 

 being present to the mind of subsequent fortunate discoverers of these remains of the 

 Hylaeosaurus, than if they were solely preoccupied by the hypothesis of the dorsal 

 fringe. For example, it may lead to more careful noting of the constancy or otherwise 



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