148 MR, R. LYDEKKER ON PART OF THE 



12. On Part of the Pelvis of Polacanthus. By E. Lydekker, Esq., 

 B.A., F.G.S. (Read December 23rd, 1891.) 



Among the specimens lately acquired by the British Museum from 

 the collection of the late Mr. Beetles, of Hastings, is one to which 

 my attention has been directed by my friend Mr. A. Smith Wood- 

 ward. It bears the number E. 1926 in the Museum Eegister ; and, 

 like the majority of Mr. Beckles's specimens, is evidently from the 

 Wealden. Mr. Charles Dawson, who has had so much experience 

 in Wealden fossils, has been good enough to examine the specimen, 

 and considers that it is almost certainly from the Isle of Wight, and 

 not from Hastings. 



One glance at the specimen is sufficient to show that it is the 

 central part of a Dinosaurian ilium, with portions of the sacral 

 ribs still attached to its inner surface. It belongs to the right side, 

 as proved by the position of the ischial tuberosity ; and while both 

 extremities of the ilium are wanting, the acetabular region is fairly 

 well preserved. The pre- acetabular portion of the ilium forms a 

 roof-like expansion ; while the post-acetabular process is compressed 

 and comparatively thin. * The upper border of the ilium is straight. 

 The broken portions of the sacral ribs (five in number) are tri- 

 angular in section, and have deep indentations between them ; so 

 that they form buttress-like structures of great strength. The 

 whole contour of the ilium and sacral ribs is quite unlike that 

 obtaining in Iguanodon and its allies. It appears most likely that 

 the pre-acetabular portion of the ilium when entire was greatly 

 expanded laterally, as is the case in Stegosaurus, 



The point of especial interest connected with the specimen is, 

 however, the presence of a large flat plate of bone, somewhat more 

 than I inch in thickness, resting on the upper border of the ilium, 

 from which it is separated by the intervention of a thin layer of 

 matrix. This bony plate is evidently a portion of a dermal armour, 

 and thus suggests comparison of the specimen with the dorsal shield 

 of the Dinosaur from the Wealden of the Isle of Wight described by 

 Mr. Hulke as Polacanthus Poxii} 



Such a comparison shows that the present specimen undoubtedly 

 belonged to a closely allied, if not specifically identical Dinosaur. 

 It is true, indeed, that in the specimen described by Mr. Hulke what 

 remains of the pelvis and sacrum is crushed almost flat on to the 

 under surface of the dorsal shield. Still, however, there is sufficient 

 preserved in the former to show that the ilium and sacral ribs are 

 of the same type as in the specimen under consideration ; the tri- 

 angular cross-section of the sacral ribs being especially noticed in 

 Mr, Hulke's description of his specimen. Moreover, when compared 

 with the pelvis and sacrum of the much larger Kimeridgian Dino- 

 saur described by Sir R, Owen as Omosaitrus, which belongs to the 

 same group as Polacanthus, the present specimen again presents a 



^ Phil. Trans, for 1887 (b), p. 169, pi, viii. [The name Tolacanthus (Hulke, 

 1881, ex Owen), as has been pointed out to me, is preoccupied by Polyacanthus, 

 Kuhl, 1831, and ought, therefore, to be changed.— Feb. 1892.] 



