R, The retinal portion of the same which, owing to the infolding, lies between gl, the corneagen or lens-forming portion, and pr, the post-retinal or capsular portion or fold.
By a process of infolding, the thicker end is partially invaginated, the middle portion or " hind-body " and the organism may now present a superficial likeness to a cercaria.
More usually, by an infolding of the layer of cells in development, we get three layers under the lens; the front layer is the corneagen layer, and is separated by a membrane from the other two which, more or less, fuse and contain the nerve-end-cells (retinal layer).
As a general rule, the depression caused by the infolding of the surface of the incisor (the " mark ") is obliterated in the first or central incisor at six years, in the second at seven years, and in the third at eight years.
Mostly large forms, of Carboniferous and Permian age, with a more or less complex infolding of the walls of the teeth.