OS PUBIS OF POLACAXTHUS FOXTI. 85. 



3 inches deep iu front. Tfcs anterior end curves a little outward, 

 as in other genera. The antero-superior margin of the blade is 

 nearly straight, compressed and sharp ; the terminal contour is 

 convex. The infero-posterior margin may be slightly broken in 

 front; it is concave in the hinder part of its length, being prolonged 

 continuously on to the post-acetabular process. 



An angular impress, ill-defined, is seen on the inner side at the 

 anterior extremity of the pre-acetabular plate, but there is no 

 specimen among the remains of Polacanthus which can be identified 

 as being the pre-pubic bone. jN'o pre-pubic bones are known in 

 Omosaurus, though it is manifest from the size and width of the 

 pelvis that the pubes could not have met each other. 



Od the whole, the pubic bone seems to be in many respects inter- 

 mediate in its characters between Omosaurus and Iguanodon. 

 ■ The ischium in its singular curved form is only paralleled by 

 Triceratops, which is stated by Baur to be identical with Agathau- 

 mus. But in Polacanthus the bone is wider, with a curious out- 

 ward process extending from the acetabulum over the ilium, shown 

 in the figure, and the pubis is dissimilar. 



The ilium of Polacanthus throughout its extent is blended with 

 the shield, so that the preservation of the shield has apparently 

 resulted from the support of the thin underlying bony framework 

 on which it rests. Possibly the definition of the ilium may have 

 become slightly more evident than when the specimen was first 

 reconstructed ; but from the acetabulum a line diverges outward 

 and forward, separating darker thicker tissue external to it from 

 the paler substance of the shield on which the ribs rest. I take 

 the great anterior triangle of darker tint to represent the thin 

 blade of the anterior process of the ilium. It is seen to be super- 

 imposed upon four of the ribs which are obscured beneath it ; but 

 there is no strong demarcation to separate the bone of the ilium 

 from the shield, and the two are blended, much like the rib and 

 costal plate of a Chelonian. Still its limits are seen. The pre- 

 acetabular process seems to form one half the length of the bone, 

 and to approximate both in shape and outward development to the 

 corresponding part of the ilium of Omosaurus. The post-acetabular 

 process was relatively short and truncated ; its form is shown in 

 fig. 2, It extends nearly to the posterior margin of the shield, 

 and, owing to removal of scutes, is seen exposed in this position 

 on the dorsal aspect in Mr. Hulke's plate. ^ 



The length of the right ilium, which is the more perfect, appears 

 to be 85 inches, but it may have extended a little farther forward. 



From this arrangement it results that, while most Ornithischia 

 have the ilia vertical, in Polacanthus they are horizontal. The 

 margins, which are usually highest here, are bent outward and down- 

 ward with the convexity of the shield so as to descend lower than 

 the acetabulum.^ Erom this condition the acetabulum has a trans- 



^ Phil. Trans. Roy. Soc. vol. clxxviii. B (1887) pi. xviii. 



2 A similar type of ilium unconnected with a shield is catalogued under 

 Cetiosaurus in the British Museum, Cat. Foss. Eept., Part I. p. 143. 



g2 



