44 



THE POPULAR SCIENCE MONTHLY. 



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 in- 

 stance, of 7i 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. (^ millim.), and the size of each compartment occupied by 

 a tooth is 28 mmm. (^ millim.), while the same measures, taken on the 

 outer edge of the valve, give 145 and 45 mmm. (^ and ^V millim.). At 

 this last point the small, wedge-shaped teeth rise to a height of 32 

 mmm. (^3 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 con- 

 tinuation. 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. ( T i-g- to ^g- 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 7^ 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 in- 

 strument better adapted than this shell for boring galleries in wood. 



