It’s an egg! A jacana laid it on some tropical riverbank or lake shore sometime in the 1800s, just so Alphonse Millot could come along and immortalize it in the pages of an encyclopedia. (Never mind that it was probably blown and stuck under glass before it was captured in watercolor. He was quite a prolific artist, Mr. Millot. At least as far as pretty things – birds, butterflies, eggs – are concerned.
While he doesn’t have much about him on Wikipedia, his pictures are pretty well-represented on Tumblr. That site is full of 19th-century hipsters, man.