320 BRITISH FOSSIL REPTILES. 



The first and most obvious objection to the fossil in question, (No. ^^, MantelUan 

 Collection, British Museum), being the bony core of a median frontal horn, is its icant 

 of si/mmetry . This is plainly manifest in two respects; first, by the obliquity of the 

 base ; and secondly, hold it as you may, by the inequality and difference of form of the 

 two sides. If the fossil be viewed with the apex upwards and forwards, as in the 

 position in which Mr. Dinkel has delineated it, PI. 22, fig. 1, when I desired him 

 to draw it in the position in which it appeared least unsymmetrical, even then the left 

 side is, b^ reason of the basal obliquity, longer than the right, and it is more convex 

 in the vertical direction. This view exposes what I believe to be the left side of the 

 phalanx. 



With respect to the base of the bone, all its natural surface, with the exception of one 

 small spot, has been chiselled or scraped away, and the central coarse cellular structure 

 of the bone is thus exposed. That single smooth spot, however, indicates that the base 

 had been articulated by a synovial joint, and the form of the rest of the mutilated 

 basal surface nowise militates against the supposition of the conical bone having been 

 tlie terminal unsymmetrical uv(/ual phalanx of the outer toe of a great Saurian reptile. 



The want of symmetry in the ungual phalanges of the outer and inner digits of a 

 reptile's foot, in which phalanges one side becomes longer and more convex than 

 the other, exemplifies the nature of that degree of want of symmetry which exists in 

 the fossil in question, and which ought of itself to be decisive against the opinion of 

 such fossil being the basis of a single median frontal horn. 



Yet this idea has been so long fixed and is so generally received, that, although the 

 objection above advanced may unsettle it, yet additional reasons may be expected 

 before it will be finally abandoned. For, to the objection of mere want of symmetry, 

 it may be replied, that this particular example of the horn of the Iguanodon may 

 exhibit an accidental deviation from the normal structure ; although, indeed, an 

 unsymmetrical horn has never been noticed in the horned Iguana {Metojmceros). Yet 

 even at this stage of the argument it will not be hard to decide between a phalanx 

 to which the unsymmetrical form presented by the fossil is natural, and a horn in 

 which such dissymmetry would be monstrous. Independently, however, of general 

 configuration there are other characters by which an unequal phalanx of a crocodilian 

 or other large Saurian may be detected. 



An ungual phalanx is a significant bone ; it has relations which no other phalangeal 

 or other bone of a foot possesses, and has modifications of surface, of form, and 

 structure subservient to those relations. 



First, it supports the strong horny sheath or claw which immediately presses upon 

 the ground, and which accordingly needs constant and copious reparation. An 

 ungual phalanx, therefore, besides its own " periosteum" is invested by a highly 

 vascular and almost glandular " corium," which is the active renovator of the worn- 

 down claw. 



