Why is the existence of the American sea-serpent no longer credited?

Because of the following exposure of its fraud, as related by Professor Silliman.

The first sea-serpent was a steam-boat, which, being established at Boston to coast along the shore, and from its powers and capabilities competent to injure the business of small boats, was described as a sea-serpent that had been seen off Nahant and Gloucester, and had probably come there to consume all the small fish in the place. This was received by many as a serious account, and believed accordingly. - Another sea-serpent history arose from the circumstance, that a small sloop, called the Sea-serpent, having been passed by another vessel, the captain of the latter, when asked, upon his arrival at home, for news, said he had seen a sea-serpent, and then described its bunches on the back, the action of its tail, and other parts; all of which being understood literally, actually appeared in print, as evidence for the existence of the animal.

Then a piece of the skin of the bony scaled pike was taken for part of a sea-serpent's hide. And from such occurrences as these, perhaps, mingled with careless observation of the motions and appearances of por-puses, basking sharks, and balaenopterous whales, appear to have originated every thing that has been said about American sea-serpents.

Dr. Fleming thinks it " probable, that many of those stories which have been propagated, regarding vast sea-snakes, have originated in the appearance of some of the larger serpents at sea, where they have been driven by accident. Some of the Asiatic species reside almost constantly in the water, either fresh or salt."

In 1827, Dr. Harwood presented to the Royal Society an account of a new serpentiform sea animal, which he named the ophiognathus. The specimen is 4 feet C> inches in length, and the jaws open wider than those of any other animal that the Doctor is acquainted with; not excepting even the rattle-snake. Its entire form indicates that it must possess great swiftness of motion in the waters.