FEOM THE KIMMEEIDGE CLAY. 447 



dered sinuous by the constriction of the middle of the centra and 

 the prominence of their terminal borders. 



The neural canal corresponding to the last lumbar and three 

 foremost sacral centra is very capacious ; at the fourth centrum it 

 becomes abruptly contracted. 



It will have been noticed that only the false sacral centrum (last 

 lumbar) wholly supports it own neural arch, and that, as regards 

 the four true sacral centra, the arch, whilst resting mainly on its 

 own centrum, is also borne in part on the centrum next before it. 

 This seeming advance of the arch, by which it comes to lie over the 

 interval between two centra (first noticed, I believe, by Owen in a 

 Wealden sacrum referred by him to Iguanodon Mantelli, formerly 

 in the collection of the late Dr. Saull, aud at his death acquired 

 by the British Museum, and also found by him in a sacrum of 

 Megaloscmras), has its probable explanation in the persistence 

 throughout life of an early embryonic phase. In the chick it has 

 been ascertained that each permanent vertebra comprises the 

 anterior and posterior halves of two consecutive protovertebrae. 

 The neural arch, after this second segmentation of the vertebral 

 column, comes to rest on the anterior half of the permanent vertebra. 

 The intervertebral space between two permanent vertebrae corre- 

 sponds to the middle of the centrum of a protovertebra. 



In the Dinosaurian sacrum it appears as if the transformation 

 of the proto- into the permanent vertebras was not completed in the 

 sacral region of the column ; the second segmentation mapping out 

 the permanent centra is effected, but the arches retain their pri- 

 mitive positions. 



On compariug this Cumnor sacrum with the type fossil in the 

 British Museum, to which I have very recently referred (No. 37685, 

 Brit.-Mus. Catal.), some notable differences are evident. Of these, 

 the smaller number of centra, 4 in the Cumnor sacrum (the British- 

 Museum sacrum has 5 centra), is the most important. It is certain 

 that 5 is the true number of centra in the latter ; for they are 

 firmly coossified in undisturbed natural sequence ; but the reference 

 of this fossil to Iguanodon Mantelli wants the confirmation which 

 its association with other indubitable Iguanodon-remains might have 

 afforded, and no such verified sacrum has since been found with 

 which to compare it. 



The proportions of the centra in the Cumnor and British- 

 Museum sacrum No. 37683 are also very different, as the subjoined 

 measurements show. In the former the centra are shorter and 

 stouter, and they want the remarkable lateral compression and 

 strongly carinate form so conspicuous in the latter, which is well 

 represented in Prof. Owen's figure of this in his Foss. Bept. Wealden 

 Formation, t. iii. 



