434 J. W. HULKE ON IGUANODON PRESTWICHII 



the length of the vertebral centra in different regions of the column, 

 and the structure of the tarsus. 



A conviction of the value of this skeleton for future reference has 

 led me to describe with greater detail than would otherwise have 

 appeared necessary all the more important and better-preserved 

 bones. 



Head (PI. XYIII.). — This has been much crushed and broken. 

 The most instructive pieces preserved are : — I. l*,the basis cranii as 

 far forward as the hypophysial fossa ; I. 9, the back of the skull, and 

 the sides nearly to the same extent as the base ; I. 10, the frontal 

 region ; I. 3, 4, 8, parts of both mandibular rami ; and I. 2, 5, 6, 7, 

 portions of both maxillse. 



1. 1 (PL XYIII. fig. 3). — The occipital condyle has a reniform out- 

 line. Its horizontal diameter is 29 millims., and its vertical diameter 

 19 millims. ; these figures, however, do not convey a correct idea of 

 the extent of the articular surface in these directions, which is greater 

 vertically than horizontally, an arrangement manifestly associated 

 with greater angular mobility of the head up and down than from side 

 to side at the occipito-atlantal joints. The condyle is composed mainly 

 of the basioccipital bone ; and only a small part at its upper lateral 

 corners is contributed by the exoccipitals. At the under surface a 

 well-marked constriction or neck separates the condyle from a 

 pair of blunt pyramidal swellings, divergent downward extensions 

 of the basisphenoid, corresponding in position to the posterior pair of 

 similarly placed processes in extant lizards. Against these basi- 

 sphenoidal swellings laterally the neck of the basioccipital expanding 

 abuts, whilst mesially it sends downwards and forwards a short 

 triangular process which is wedged in between them. This inter- 

 calated basioccipital process has a low but sharp median crest, 

 which begins behind in a small pit, pierced by vascular foramina 

 lying in front of the condyle. 



When the front of the fossil is viewed there are apparent, 

 Ist, below, in the middle line, a rough broken spot, about 12 millims. 

 in diameter, where the prsesphenoid has been split off ; 2nd, above 

 this a smooth vertical groove, ending above in the floor of the 

 cranial cavity — it is obviously the posterior half of the hypo- 

 physial pit ; 3rd, on each side of this an uneven fractured boss, the 

 junction of ali- and basisphenoids.. Each of these bosses overhangs 

 the smooth antero-lateral surface of the descending basisphenoidal 

 process of its own side ; and there is immediately beneath it a 

 winding groove, which rises from the undersurface of the skull 

 through the hypophysial fossa into the cranial cavity ; it marks the 

 course of the internal carotid artery. A branch of this groove, 

 ascending in a backward direction, enters the skull through the 

 principal foramen of exit for the 5th nerve. 



PI. XVIII. fig. 4. The upper surface of the fragment exhibits (1) a 

 mesial trough, the floor of the skull-chamber. This is 7*5 millims. 

 wide near the foramen magnum ; and here it is relatively deep. An- 

 teriorly it widens through a space of 26 millims., and then deepening 

 * These numbers refer to labels attached to the bones. 



