in fact the only portion which exists at a very early period, it follows that the average dimension of the teeth increases with the size of the shell, that is to say, with the growth of the animal. On a shell, for instance, of 72 millim. in its largest dimension, where the total number
|Fig. 7.||Fig. 8.|
of rows of teeth reaches 41, the width of each row near the hinge-part is 52 mmm. (19 millim.), and the size of each compartment occupied by a tooth is 28 mmm. (36 millim.), while the same measures, taken on the outer edge of the valve, give 145 and 45 mmm. (7 and 22 millim.). At this last point the small, wedge-shaped teeth rise to a height of 32 mmm. (33 millim.) above their common support. On an average, there are in each row about 100 teeth, and consequently more than 4,000 on each valve, and more than 8,000 on the two valves together.
The anterior part, in the form of a spoon, has a similar structure, but still more delicate. The lines of growth form an angle of a little more than 90° with those of the middle part, of which they are a continuation. They appear like small, projecting ribs, the outer edges of which are cut in the form of small teeth pressed one against the other (Fig. 6, c, and Fig. 9). These denticles are also in form of wedges; their cutting surfaces are perpendicular to the axis of the animal, and consequently form a right angle with the cutting surfaces of the teeth of the middle part of the shell. But they are much smaller than the latter; their width is only 10 to 15 mmm. (100 to 66 millim.). Their number is also more considerable, even although that part of the shell is less fully developed than the rest.
On the same shell of 72 millim. diameter, the number of these denticles is, on an average, 250 on each rib, which makes 10,250 on the 41 ribs, and 20,500 on the two valves.
We should also point out the fact that this spiral part of the shell is evidently composed of more solid matter than the rest of the shell. It has more lustre, and the look of porcelain, and its surface between the ribs is smooth and glossy.
The consideration of the structure which we have related led Mr. Harting to the conclusion that it would be difficult to imagine an instrument better adapted than this shell for boring galleries in wood.