=Mantle Reflected over the Shell.=--It is sometimes the case that the mantle edges tend to double back over the external surface of the shell, and to enclose it to a greater or less extent. When this process is carried to an extreme, the edges of the reflected mantle unite, and the shell becomes completely internal. We see an incipient stage of this process in _Cypraea_ and _Marginella_, where the bright polish on the surface of the shell is due to the protection afforded by the lobes of the mantle. A considerable portion of the shell of _Scutus_ is concealed in a similar way, while in _Cryptochiton_, _Lamellaria_, and _Aplysia_ the shell is more or less completely enclosed. Among Pulmonata, it is possible that in forms like _Vitrina_, _Parmacella_, _Limax_, and _Arion_, we have successive stages in a process which starts with a shell completely external, as in _Helix_, and ends, not merely by enveloping the shell in the mantle, but by effecting its disappearance altogether. In _Vitrina_ and some allied genera we have a type in which the mantle lobes are partly reflected over the shell, which at the same time exhibits rather less of a spiral form than in _Helix_. In the stage represented by _Parmacella_, the mantle edges have coalesced over the whole of the shell, except for a small aperture immediately over the spire; the nucleus alone of the shell is spiral, the rest considerably flattened. In _Limax_ the shell has become completely internal, and is simply a flat and very thin plate, the spiral form being entirely lost, and the nucleus represented by a simple thickening at one end of the plate. In _Arion_, the final stage, we find that the shell, being no longer needed as a protection to the vital organs, has either become resolved into a number of independent granules, or else has entirely disappeared. Some indications of a similar series of changes occur in the Pelecypoda. The mantle edge of _Lepton_ is prolonged beyond the area of the valves, terminating in some cases in a number of filaments. In _Galeomma_ and _Scintilla_ the valves are partially concealed by the reflected mantle lobes, and in a remarkable form recently discovered by Dall[282] (_Chlamydoconcha_) the shell is completely imbedded in the mantle, which is perforated at the anterior end by an orifice for the mouth, and at the posterior end by a similar orifice for the anus. In all these cases, except _Lepton_, it is interesting to notice that the hinge teeth have completely disappeared, the additional closing power gained by the external mantle rendering the work done by a hinge unnecessary. It is quite possible, on the analogy of the Gasteropoda mentioned above, and also, it may be added, of the Cephalopoda and other groups, that we have here indicated the eventual occurrence of a type of Pelecypoda altogether deprived of valves, a greatly thickened mantle performing the part of a shell.[283] * * * * *