234 



body, at its utmost length ; it would then be only to leave the tube to 

 its own powers, when, by its elasticity, it would coil up to its original* 

 form, and drag the shell to the point to which it was attached. I could 

 easily devise other uses for this part, but am unwilling to wander too far 

 into the regions of conjecture. 



I must here observe, that as this curious structure filled nearly one 

 half of the shell, I was anxious to discover whether a similar structure 

 existed on the other side ; but this side was so completely iilled with spar, 

 that after having very much injured the specimen, the fear of destroying 

 it entirely made me desist; not, however, until I had found very great 

 reason for believing that a similar body did not exist on that side, 

 although traces of some kind of organization might here be discerned. 



By viewing the long patulous hinge of the shell Plate XVI. Fig. 11, 

 and supposing a large trigonal aperture to exist between the beaks, 

 you obtain a correct idea of the characters of that family of shells which 

 Mr. Martin describes as being perforated, having both valves convex, 

 with a straight open hinge, and a large trigonal foramen between the 

 beaks. But the different species of these shells differ much among them 

 selves, and particularly in the depth of the area belonging to the hinge. 



The shell represented Plate XVI. Fig. 17, shows this area, spread 

 out to a surprising extent. This shell was first noticed by Mr. Martin, 

 in the Linnean Transactions, Vol. iv. p. 4 ; and, since that, in Pefri- 

 ficata Derbiensia, Plate 46, where he thus describes it : " A petrified 

 shell. The original an Anomia. Perforate. Valves convex. Hinge 

 straight, extended, patulous, triangular, divided down the middle by 

 a very large perpendicular foramen, in form of an acute isoceles. The 

 surface of the shell longitudinally furrowed. The furrows strong, about 

 28 or 30, crossed by a few distant wrinkles, marking, apparently, the 

 growth of the shell. A deep, rounded sinus, destitute of furrows, in the 

 larger valve : answered by a convex wave on the other valve, and ter- 

 minated by a strong curvature at the margin. Perforated valve, pyra- 

 midal, perpendicular, gradually tapering from the margin to the beak, 



