These are _conical_, _irregular_, _spiral_, or _convolute_. The _conical_ form is when there is no enrolment of the apex. Although the Patellæ were described as symmetrical, there are several species of Patelliform shells which are not symmetrical. In Umbrella, for instance, the apex is oblique, the shells being placed obliquely on the animal. In the genus _Siphonaria_, there is a groove on one side, where the brachia or gills of the animal rest. In the genus Ancylus, it will be observed that the apex bends on one side, and the animal is like the Limnæa, which has a spiral shell. The cup and saucer Limpets, or Calyptrædæ, present a group which requires to be described, differently from the symmetrical or true Limpets. Their structure is very curious, and they vary considerably among themselves, some of them being simply conical, others nearly flat, or discoidal, and others more or less spiral. But their principal peculiarity consists in their having a small internal process or plate variously shaped, commonly named their _septum_. _Septa of Limpets._ The septa of Limpets assume a variety of forms, the principal of which will be seen in the accompanying engravings. [Illustration] The form from which the group derives its generic appellation is that of the cup-shaped or _Cyathiform_ species (fig. 17). In the Crepidulæ, or Slipper-Limpets, the septum is flat, reaching across the opening, like the deck of a vessel; it is then described as _transverse_ (fig. 20). In Calyptræa Equestris, it has two prominent points, and is described as _bi-furcated_ (fig. 18). In another species, it is a three-sided plate rather spiral at the apex (fig. 19). _Measurement of Cup and Saucer Limpets._ [Illustration] The line marked _a_, _p_, _ll_ indicates the direction in which the shell is to be measured for _length_. _a_ indicates the _anterior_, _p_ the _posterior_. The line _d_ (fig. 23), from the apex to the base, is the _depth_. The line _b_ (fig. 28), is in the direction of the breadth. _Irregular non-symmetrical Univalves._ Serpuliform shells are irregularly twisted (_tortuous_) hollow tubes, which were formerly considered to have been secreted by a kind of worm, but now known to be the shells of true Molluscs, of a kind not very widely differing from those which have regularly spiral shells. The greater part of these are attached to foreign bodies, or to each other in groups. Some are attached by the whole length of the shell, they are then said to be _decumbent_. Some of these are coiled round like the Spirorbis, the little white shell seen on the carapace of the Lobster or on leaves of sea-weeds; they are then said to be discoidal; others again, such as the _Vermetus_, approach more nearly to the spiral form. The deviation from the regular spire only taking place after the few first volutions. SPIRAL NON-SYMMETRICAL UNIVALVES. As these constitute the largest class, it will be necessary to dwell upon them in detail. First as to _measurement_.