WEALDEN FORMATIONS. 15 



Caudal Vertebra. T. X. 



A proportion of the tail, to the extent of nearly six feet, and including about 

 twenty-six vertebrae, discovered in a quarry in Tilgate Forest in the year 1837, is 

 preserved in the British Museum (T. X). The diapophyses (d, d) present almost 

 Crocodilian proportions, in regard to their length, at the interior part of this series, 

 and may be discerned, though diminished to mere rudiments, in the small terminal 

 vertebras of the series. In the most perfect of the anterior vertebras they are com- 

 pressed vertically, but with convex, not flattened sides, and rounded edges, presenting 

 an elliptical transverse section, and preserving the same breadth to their truncated 

 extremity : they extend outwards, and are slightly bent forwards : the breadth of 

 this vertebra between the extremities of the transverse processes is 1 1 inches. The 

 neurapophysis is curved forwards from the base of the diapophysis to form the prezy- 

 gapophysis, or anterior oblique process : its length from the extremity of this process 

 to that of the posterior one is 3^- inches. The neurapophysis presents a simple convex 

 external surface up to the base of the neural spine ; the antero-posterior extent of this 

 process is 2 inches. The haemal arches are from 4 to 5 inches in length near the base 

 of the tail (figs. 3 and 4) ; they may be distinguished, like the diapophyses, by their 

 convex external surface ; their bases come into contact, as shown in fig. 4, but are not 

 confluent as in the Iguanodon ; they articulate to two separate hypapophyses. 

 Between the pairs of these tubercles, which are placed at each end of the under 

 surface of the centrum, there is a longitudinal sulcus. The diapophyses soon lose the 

 slight anterior curve, stand straight out, decrease in length, and descend from the 

 neurapophysis to the centrum as the vertebrae approach the end of the tail. 



The haemal arches also decrease in length, but they expand in the antero-posterior 

 direction at their unattached and dependent extremity, which is defined by a slight 

 convex outline. Fig. 4 shows the modification of the under surface of the caudal 

 vertebrae, at the middle of the tail ; and fig. 5 gives a side view of one of the haemal 

 arches from this part, of the natural size. The following admeasurements give the 

 rate of decrease in length in the caudal vertebrae, taken at intervals of six joints : 



In. Lin. 

 Length of body of presumed 8th caudal . . . . . 2 G 



Length of body of presumed 14th caudal . . . . .24 



Length of body of presumed 20th caudal . . . . .22 



The sides of the slender posterior vertebrae are distinguished by a slight median 

 expansion below the base of the rudimental transverse process, so that the surface, 

 instead of being gently concave lengthwise, undulates by virtue of the middle elevation. 



