GENUS CHELONOBIA. 391 



appears more wonderful even than it really is. The younger 

 shells have the appearance of having grown from within the 

 carapace, and then of having burst through it, almost like 

 little volcanos. I have seen only one very young shell (^th 

 of an inch in external diameter) attached, and here there was 

 nothing to countenance an idea which at one time occurred 

 to me, namely, that the larva perhaps fixed itself in some 

 little crack or cavity in the carapace, and there underwent 

 its metamorphosis. I believe that the imbedment is effected 

 simply by the sharp, growing, basal edges of the walls of 

 the shell indenting the surface, and finally rupturing the 

 outer laminae of the tortoise-shell, through that same force 

 by which the tender radicle of a plant penetrates very- 

 hard ground. As soon as the surface was once rup- 

 tured, the shell of the Chelonobia, growing outwards and 

 downwards, would easily, like a wedge, turn up the laminae 

 of the tortoise-shell ; and their ragged ends would surround 

 the Chelonobia, as is seen actually to be the case. In the 

 genus Coronula and its allies, which are attached to Ceta- 

 ceans, we shall presently see, that the epidermis imme- 

 diately under the downward growing shell, and apparently 

 in consequence of the pressure thus exerted, is thinner than 

 m the surrounding parts. In two specimens of Chelonobia 

 caretta, imbedded much more deeply than usual — in one of 

 which half the basal edge of the shell had fairly cut 

 through the carapace, and in the other was on the point of 

 effecting this — the tortoise-shell manifestly thinned out to- 

 wards the line of yielding; now this, I suppose, must be 

 attributed either to absorption, or to the living tortoise- 

 shell being actually stretched till rendered transparent and 

 ready to burst or until bursted. On the latter view of the 

 tortoise-shell having been stretched, we must further suppose 

 that the pressure has prevented fresh layers of tortoise- 

 shell being deposited under the old and yielding laminae. 

 In one of the above two specimens, the walls of the Chelo- 

 nobia were deeply folded, nevertheless the laminae of the 

 tortoise-shell followed every curvature, showing that, though 

 now so rigid, during the slow imbedment of the Cirripede it 

 must have been sufficiently pliant. A shell attached, as these 

 two specimens were, could never be removed, and, whether 



