THE OS PUBIS OF POLICAXTHTJS FOXn. 81 



7. 0)1 the Os Pubis 0/ PoLACAiN^rHus Foxii. By H. G. Seelet, Esq., 

 P.H.S., F.G.S., Professor of Geography in King's College, 

 London. (Read I^ovemher 2d, 1891.) 



The evidence of the systematic position of the Wealden fossil 

 Polacanthus has not been very precise. Mr. Hulke in 1881 re- 

 garded the dermal armour as closely resembling that of Scelido- 

 saurus ; and the dermal spines were compared with those of Acan- 

 ihopJiolis, Stegosaurics, and Omosaiirus, with indications of some 

 differences of structure. It is to the Wealden Hylceosaurus that 

 Polacanthus was most closely allied, the resemblance in their dermal 

 spines being very close, and their tibiae are stated to be remarkably 

 alike. These views were adopted by Prof. Marsh, who in January 

 1882 placed Polacanthus in the Scelidosauridae, immediately after 

 Hylceosaurus. 



When the great tuberculate shield which covers the pelvic region 

 in Polacanthus became known in 1887 by the skill of Mr. J. Lingard 

 in fitting it together and of Mr. Kichard Hail in removing the 

 matrix, Mr. Hulke described the pelvis,^ and found the ilia so 

 blended with the dermal bones that their exact form was not made 

 ont.^ The remains of the os pubis are said to be too fragmentary 

 to give the shape of that bone ; but it was thought that indications 

 were recognizable of its division into posterior and anterior parts. 

 I was unable to recognize any indication of that bone on the shield, 

 and the lettering on Mr. Hulke's figure only refers to the ischium 

 and acetabulum. Further search fortunately resulted in detecting 

 the missing pubis as an isolated specimen, collected by Mr. Pox 

 with the other remains, east of Barnes Chine, in the Isle of Wight, 

 in 1865. It appears to be indicated in Mr. Lydekker's ' Catalogue 

 of Fossil Reptiles in the British Museum,' Part I. p. 190, by the 

 description " a dermal scute bearing a short spine." This bone I 

 regard as the anterior portion of the left pubis, the short spine 

 being the proximal portion of the post-pubis ; together they prove 

 that the pubis in Polacanthus, while showing diff'erences, was con- 

 structed substantially upon what may be termed " the Iguanodont 

 plan." The most obvious difference is that the anterior pre-aceta- 

 bular portion is relatively shorter than in any genus hitherto 

 described. 



The thin anterior plate of the pubis, which was vertical and 

 compressed from side to side, thickens as it extends backward to 

 form the superior articular surface for the ilium, the margin of the 

 acetabulum, and the small inferior articulation with the ischium, 

 which was not sutural. 



From its inferior internal side is given off the so-called post- 



^ Phil. Tra,ns. E-oy. Soc. vol. clxxviii. B, pis. viii., ix. 



2 The outhne of the ilium is shown in pi. ix. {op. cit.), except that the inner 

 anterior border of the bone is less clearly defined in the drawing than in the 

 specimen. 



Q. J. G. S. ^"0. 189. G 



