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At the new salt-works, and at the village of Maniquarez, these stones of the eyes* were offered to us by hundreds, and the natives were anxious to show us the experiment of the lemon juice. It was easy to see that the stones are thin and porous opercula, which have formed part of small univalve shells. Their diameter varies from one to four lines.
A still more highly endowed relation spins a similar fabric, upon which are loosely agglutinated numbers of small dead shells, grit, and even opercula a quarter of an inch in diameter. In weight, size, and number of its constituents this exterior armour is altogether disproportionate to the extreme tenuity of its foundation.
On April 12, when they had all passed the stage of external gills and developed internal gills and opercula, I divided them into two lots, one in a shallow pie-dish, the other in a glass cylinder. To one lot I gave a portion of rabbit's thyroid, to the other a piece of rabbit's liver. They fed eagerly on both. Afterwards I obtained at intervals of a week or so the thyroid of a sheep.
One of their two surfaces is plane, and the other convex. These calcareous opercula effervesce with lemon juice, and put themselves in motion in proportion as the carbonic acid is disengaged. By the effect of a similar reaction, loaves placed in an oven move sometimes on a horizontal plane; a phenomenon that has given occasion, in Europe, to the popular prejudice of enchanted ovens.
The smallest of them was in total length 1.7 cm. body, 0.7 " tail, 1.0 " The largest measured was in total length 2.2 " body, 0.8 " tail, 1.4 " May 25. All but two of the tadpoles dead. The tails were only half the original length, all had well-developed hind-legs, some with toes, but the fore-legs were beneath the opercula, not projecting from the surface.
But, as a general rule, the fluviatile species are smaller, smoother, and more globular than the marine; and they have never, like the Neritae, the inner margin of the outer lip toothed or crenulated. Potamides cinctus, Sowerby. The Potamides inhabit the mouths of rivers in warm latitudes, and are distinguishable from the marine Cerithia by their orbicular and multispiral opercula.
But the difficulty in accepting this conclusion was that there was a distinct though gentle respiratory movement of the jaws and opercula; and if the pumping of the water from beneath the body caused a negative pressure there, and a positive pressure on the outer side of the body, it seemed equally certain that the respiratory movement must force water into the space beneath the body and so cause a positive pressure there which would tend to force the fish away from the surface with which it was in contact.
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