the hood in Ammonites. The central area of the hood in Nautilus, supposed to dorronpond with

Fig 3

Fig 2

Fig 1.

The matrix which occupies the space supposed to have once been filled by the soft parts, is a very fine whitish marl, a mixture of lime and clay, softer than the grey clay of the ordinary external matrix. Much of this whitish marl may have been washed away during the years of exposure.

If, then, organs or coverings of the animal, at all analogous to the hood and mantle of the Nautilus, existed in the living Ammonite, it may reasonably be suggested that the margins of these organs or coverings might be of a much firmer texture than their middle portions; and, if so, that the margins would fossilise into a more durable substance. When, therefore, denudation took place, the marginal part might resist dissolution, and be left standing out as a prominent ridge, such as is seen in Plate I., Fig. I. (b), (c) and (d). The question next for consideration was whether the hood of the Nautilus would present anything like such a margin.

To answer this, with the concurrence and assistance of Mr. Moore and Dr. Herdman, it was resolved to remove a portion of the shell of a Nautilus containing the animal, so as to show the external appendages of the body. Further dissection was needless, because the Turvey Ammonite gave no indications of any internal structure wherewith to form a comparison.

Our Nautilus, thus displayed, gave full confirmation to the results of Professor Owen's researches. (Memoir, 1832). The funnel was seen close within the external convex wall of the shell. When the shell was first cut, the posterior fold of the mantle was visibly in contact with the involute portion of the shell, where, in many examples, the shell is coloured black. Notwithstanding the precautions used, a certain amount of shrinking occurred, and the mantle-fold disappeared below the hood.

The hood exhibits three portions, all of which are

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