Actual observation in the aquarium has, however, proved that the second hypothesis is the true one. M. Cailliaud in France, and Mr. Robertson in England, have demonstrated that the Pholas uses its shell as a rasp, wearing away the stone with the asperities with which the anterior parts of the valves are furnished. Between these gentlemen a somewhat hot contention was maintained for the honour of priority in this valuable discovery. M. Cailliaud himself used the valves of the dead shell, and imitating the natural conditions as well as he could, actually bored an imitative hole, by making them rotate. Mr. Robertson at Brighton exhibited to the public living Pholades in the act of boring in masses of chalk. He describes it as “a living combination of three instruments, viz., a hydraulic apparatus, a rasp, and a syringe.” But the first and last of these powers can be considered only as an accessory to the removing of the detritus out of the way when once the hole was bored, the rasp being the real power. If you examine these living shells you will see that the fore part, where the foot protrudes, is set with stony points arranged in transverse and longitudinal rows; the former being the result of elevated ridges radiating from the hinge, the latter that of the edges of successive growths of the shell. These points have the most accurate resemblance to those set on a steel rasp in a blacksmith’s shop. It is interesting to know that the shell is preserved from being itself permanently worn away by the fact that it is composed of arragonite, a substance much harder than those in which the Pholas burrows. Yet we see by comparing specimens one with another, that such a destructive action does in time take place, for some have the rasping points much more worn than others, many of the older ones being nearly smooth. [Illustration: PHOLADES IN A BLOCK OF GNEISS.]