100 



fractions produce sometimes additional folds and elevations. (PL. iv. fig. 1. 

 to 1 1.) If any doubt should still be entertained whether these folds and mark- 

 ings on the plates are really derived from muscular contraction, on the ground 

 that they may have possibly resulted from the original configuration of the solid 

 calcareous matter which forms them ; a general view of their phenomena, and 

 change of figure under various circumstances, in a manner which may be readily 

 accounted for on the hypothesis of their muscularity, but not on any other, 

 will, I am persuaded, remove these doubts ; but I have it in my power to 

 adduce a proof still more decisive. There are some costal plates in rny pos- 

 session which appear to have been torn from the animal with violence while 

 living, or before decomposition took place. The consequence of this would 

 naturally be that the lacerated muscle would shrink from the edges of the 

 plates towards its point of adhesion in the centre ; and accordingly this is 

 exactly the appearance preserved in the fossil specimens ; the surface where 

 denuded of its muscle, being irregularly corrugated, (?L. m.fig. 15. 16. and 

 20.) whilst the muscular folds are gathered into the centre. 



As the pelvis supports six plates, it might have been reasonably concluded 

 that it was the intention of nature to sustain six arms ; and as the animal has 

 actually only five, that the omission must either create a great vacancy, 

 or give rise to a necessary alteration of the general mechanism, in order to 

 render the circular net, formed by the arms and fingers when extended, com- 

 plete. Nature however is never at a loss, but accomplishes her purposes with 

 ease, notwithstanding the new difficulties that are created by continually vary- 

 ing structures. Thus in this animal the apparent difficulty so presented is 

 obviated by the change of form in the supernumerary costal which is penta- 

 gonal, while the other five are hexagonal. From the general arrangement of 

 the plates, these five hexagonal costals can only give rise to a series of second 

 costals, terminating in scapulae and arms, (PL. u. fig. 1.) while the irregular or 

 sixth costal plate intervening between them, (PL. n. fig. 2.) having a pentagonal 

 form, and presenting only the edges of its upper angle to the next row, supports 

 two series of intercostal plates occupying the interval, occasioned by its interpo- 

 lation, and so formed that this interval in the succeeding rows gradually 

 diminishes in proportion to the whole circumference, so that although there 

 is still a somewhat greater distance here between the two scapulae placed on 

 either side of it, than between the other scapulae, yet the irregularity is not such 

 as to occasion any material inconvenience or interruption of symmetry. The 



