116 REPORT — 1841. 



crease somewhat rapidly in length, the second being fourteen inches long and 

 the third eleven inches long ; but slightly increase in breadth. 



These remarkable bones are regarded by Dr. Mantell* as having formed 

 part of a serrated fringe extended along the back of the animal, analogous to 

 that of the Cycliira Lizard. This ingenious suggestion carries with it so 

 high a degree of probability, that I had not thought of comparing the bones 

 in question with any other part of the skeleton, until my attention was arrested 

 by observing 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 conca- 

 vity towards the observer, which, however, may be paralleled by a similar con- 

 cavity on the opposite side buried in the stone ; but the anterior or convex 

 margin inclines from the middle line towards the concave side. With regard 

 to their relative position to the rest of the skeleton, it must be observed that 

 the ventral surface of this is exposed ; so that the under parts of the bodies of 

 the vertebrae are towards the observer, and their spines imbedded in the matrix. 

 The coracoid and scapular arch are placed, as might be expected in a ske- 

 leton little disturbed and lying on its back, with their under surfaces towards 

 the observer, and covering, like a buckler, a portion of the vertebrae 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 characterize 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 use- 

 ful to compare the long flattened bones in question, as well as with the sup- 

 porting bones of a dorsal crest, in the event of a future discovery of a skeleton 

 or portion of skeleton of the Hylceosaurus including these bones. The objec- 

 tion to their being abdominal ribs, which may be founded on their great rela- 

 tive breadth as compared with those ribs in other Saurians, and especially 

 with the vertebral ribs of the Hylceosaiirus itself, deserves due consideration ; 

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

 superadded spines in the Lizard with a dorsal ft-inge, or with the spines of the 

 vertebrte themselves in the HylcBosaiirus. For the dorsal dermal spines in 

 the Cyclura correspond in number with the spines of the vertebrae which sup- 

 port them, while the base of each of the hypothetical dermal spines of the Hy- 

 laeosaur extends over more than two vertebrae. 



In the Monotremata ( Ornithorhynchus and Echidna) the abdominal ribs 

 are as much broader than the vertebral ribs as they would be in the Hylcco- 

 saurus, on the costal hypothesis of the detached bony plates here suggested ; 

 and, after the close repetition, in the Ichthyosaurus, of another of the remarka- 

 ble 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 inonotrematous 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 aUuded 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 disar- 

 ticulation of the entire skeleton 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, witii due diffidence, a choice of opinions respecting the nature 

 of these singular bones, I have been actuated solely with the view of accele- 

 rating the acquisition of the true one, which, it is obvious, will be more likely 

 * Geology of South-east of England, p. 323. Wonder* of Geology, vol. i. p. 402. 



