PUNCTURELLA. 475 



tudinal chink or crevice, which is not of equal breadth 

 throughout, but tapers above, chiefly enlarges near its lower 

 extremity, and then again slightly contracts ; it never 

 approaches the base of the shell, being distant from it, in 

 the adult, at the least two-thirds of the length of the front 

 area. It communicates with a small square-mouthed funnel- 

 shaped chamber, whose walls are detached at the broader 

 end, but are strengthened at their commencement by a 

 transverse lamina of shelly matter. The margin or aperture 

 of the shell is more or less oval, rounded at the extremities, 

 and tolerably convex at the sides ; it is crenately undu- 

 lated by the external sculpture. This last consists of 

 numerous radiating narrow costellse, that are alternately 

 larger and smaller, and are rendered rough, and, in the 

 young, and toward the vertex of the adult, subgranular, by 

 a fine and crowded concentric decussation. 



One of our largest examples measures four lines in 

 breadth, and rather exceeds five in length. 



The animal is white. The head, Avhich is tumid but 

 short, bears two rather obtuse subulate stout tentacula, 

 with the eyes, which are very large, on prominent bulg- 

 ings, or short peduncles at their external bases. The 

 mantle is simple-edged. The foot is oblong and not steep 

 or high-sided ; at its junction with the body there are, on 

 each side, six or seven short cirrhi, and an odd one, larger 

 than the rest, and behind them on the left side. There 

 are no cirrhi posteriorly. From the anal cleft projects a 

 conspicuous truncated sheath-like membrane, open in front, 

 where there are three or more retractile papillae. The 

 branchiae are distinctly visible in the cavity behind the 

 head. 



This very curious shell was first observed as a fossil by 

 Linnaeus himself, in the pleistocene beds of Sweden. Not 



