These ova are deposited, according to M. Guérin-Méneville, by Hemipterous (Notonectid) insects; the most abundant being the Corixa femorata; the other (which lays the larger eggs) being the Notonecta unifasciata. They are said to fly about the water in myriad's, and occasionally plunge below the surface to the depth of several feet, there depositing the eggs. The Indians collect the eggs, by placing bundles of reeds in the water, which in twelve or fifteen days get covered with millions of ova '; these they dry for an hour or two, and remove from the rushes, and sell as an article of food.[1]
M. Virlet d'Aoust remarks that probably an analogous origin of oolite has existed in all geological epochs; and he points out some of the conditions of certain oolites that favour this view,—indeed, he seems to think that nearly, if not quite, all oolites, calcareous and ferruginous, have been formed in this way.[2]
The oolitic travertine of Tezcuco is also described by Mr. E. B. Tylor, in the interesting narrative of his travels in Mexico, entitled 'Anahuac; or, Mexico and the Mexicans, Ancient and Modern,' 1859, already reviewed in the 'Geologist.' At page 156, he says:—
"When I look through my notes about Tezcuco, I do not find much more to mention, except that a favourite dish here consists of flies' eggs fried. These eggs are deposited at the edge of the lake, and the Indians fish them out and sell them in the market-place. So large is the quantity of these eggs, that at a spot where a little stream deposits carbonate of lime, a peculiar kind of travertine is forming, which consists of masses of them imbedded in the calcareous deposit. The flies which produce these eggs are called by the Mexicans 'axayacatl,' or 'water-face.' There was a celebrated Aztec king who was called Axayacatl; and his name is indicated in the picture-writings by a drawing of a man's face covered
