The Siphon is _dorsal_ when placed near the outside of the whorls; _central_ when near the middle; and _ventral_ when near the inside of the whorl, or that part which leans against the last volution. When it passes uninterruptedly from one chamber to another, it is described as _continuous_, as in the case of Spirula; when, on the other hand, it only passes through the septum a little distance, and opens into the chamber, as in Nautilus, it is _discontinuous_. _Whorls of Symmetrical Univalves._ They are _disunited_ when they do not touch each other, as in the case of Spirula (fig. 471 in the plates); but in the contrary case they are said to be _contiguous_. In some species of Nautilus the whorls overwrap each other in such a manner that the early whorls are entirely covered by the last, the edges of which reach to the centre of the disk: the spire is then said to be _hidden_; as in the Nautilus Pompilius. In Nautilus umbilicatus the spire is nearly hidden, the whorls not quite covering each other; but in the greater number of the Ammonites, the largest part of the preceding whorls is seen. To express the degree in which the whorls overwrap each other, has caused much difficulty in concise descriptions. Perhaps it would be well to apply the term _spiral disc_ to so much of the shell as is seen besides the last whorl, and to describe it as large or small in diameter, compared with the whole: or to say that the whorls of the spire are half, or one-third, or one-fourth covered, as the case may be. _Aperture of Symmetrical Univalves._ In Ammonites Blagdeni and some others the aperture is of an oblong square; it is then said to be _sub-quadrated_; in Nautilus triangularis it is _angulated_; in Ammonites Greenoughi it is of an interrupted oval shape, described as _elliptical_. In the greater number of Orthocerata, it is rounded or _circular_. The entrance of the last whorl into the aperture of some rounded species of Nautilus causes it to take a _semi-lunar_ form; if rounded at the sides it is said to be reniform or kidney-shaped; if pointed at the sides it is _semi-lunar_; and in some species of Ammonites, it is five-sided or _quinque-lateral_. _Measurement of Symmetrical Conical Univalves._ [Illustration] The _width_ is measured across the aperture, which is the widest part of the shell. The _length_ (_l_) from the dorsal part (_d_) of the aperture to the dorsal part of the _whorl_ (_d_) on the opposite part of the shell. The _ventral_ part of the whorls is that nearest to the axis, and the _dorsal_ that which forms the outline of the figure. NON-SYMMETRICAL UNIVALVES.