24 FOSSIL REPTILIA OF THE 



Lizard. The chief objection, though not decisive, against this view is, a want of 

 symmetry in the form of the most perfect of them. They are nearly flat, but along 

 the middle present a slight degree of concavity towards the observer, which, however, 

 I once thought " might be paralleled by a similar concavity on the oposite side buried 

 in the stone ;"* but a separate specimen since obtained proves that side to have 

 been convex (T. IX, fig. 3) ; and the anterior margin in the bones (d, d, T. IV) inclines 

 from the middle line towards the concave side. 



With regard to their relative position to the rest of the skeleton, it must be 

 remembered that the ventral surface of this is exposed (T. IV) ; so that the under 

 parts of the bodies of the vertebra? 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 ribs, 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 Hylaeosaurus 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 Hylaeosaurus 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 Cycluar 

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

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

 than two vertebrae. 



In the Monotrematous quadrupeds (Ornithorhynchus and Echidna) 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. 



* « 



Reports of British Association,' 1841, p. 116. 



