424 BRITISH FOSSIL REPTILES. 



agrees with Poikilopleuron and differs from Iguanodon, in which the neural arch is 

 anchylosed with the centrum, but evident traces of the suture remain, at least in 

 the dorsal vertebrge. The anterior part of the side of the centrum is impressed by 

 a large surface for the head of the rib ; the surface is concave in the axis of the 

 vertebra, convex vertically, and is bounded above by a well-defined ridge. 



The anterior zygapophysis (figs. 1 and 2, «) supports flat, articular surfaces, of an 

 elliptical form, IG lines by 9 lines, looking upward and inward ; the lower edges of 

 the pair of surfaces converging at an angle of 50°. These edges are separated from 

 each other by a fissure 3| lines broad, continued to the base of the anterior oblique 

 processes. In the Iguanodon the corresponding surfaces incline to each other at a 

 right angle, and the lower margins of the processes are united by a continuous tract 

 of bone. Each anterior articular surface is supported by a stout process, convex 

 externally, inclining forward and slightly expanding, so as to overhang and extend 

 beyond the anterior end of the centrum. A deep and narrow excavation commences 

 immediately behind tlie upper and posterior origins of the anterior zygopophyses, 

 and is continued backward, increasing in vertical extent, deep into the anterior part 

 of the base of the spinous process. Immediately behind the columnar portion of the 

 zygapophysis a conical cavity sinks into the neurapophysis, undermining the anterior 

 part of the base of the diapophysis, and dividing the zygapophysis from it. 



The diapophysis commences from the summit of the neurapophysis immediately 

 exterior to the anterior part of the base of the spinous process, by a ridge which is 

 continued backward from the upper and outer margin of the anterior zygopophysis, 

 in a gentle curve outwards and slightly upwards. The posterior margin of the base 

 of the diapophysis is not continued, in like manner, into the posterior zygapophysis, 

 but terminates or sub.^ides into a ridge above, and separated from that process by 

 a wide groove. 



The bases of the two diapophyses are only separated from each other, owing to 

 the modification of the neural arch above mentioned, by a thickness of bone not 

 exceeding 4 lines; the interspace of the origins of the two diapophyses in a corre- 

 sponding vertebra of the Iguanodon measures 4 niches ; the leugtli of the base of 

 the neural arch being the same in the vertebra compared. 



The antero-posterior extent of the base of the diapophysis in this (presumed) 

 vertebra of the Poikilojjleuron is 2 inches 2 lines ; the length of the diapopiiysis is 

 4 inches ; the vertical diameter, or thickness of the same process, where unsupported^ 

 is from 2 to 3 lines. It is obvious, therefore, that this long, thin, lamelliform plate of 

 bone must need lurther support, in order to sustain the rib which is appended by its 

 tubercle to the extremity ; and the requisite strength is here given precisely as the 

 carpenter supports a shelf by a bracket. The bracket-like process (fig. 5, c) is a 

 vertical, triangular plate of bone, the breadth or depth of which, at its origin, is 

 1 inch 4 hues ; this gradually diminishes in depth and increases in thickness as it 



