perforate the mantle, and therefore cannot be directly fixed into the shell; they are, however, connected with it through the medium of thin filmy layers of a corneous texture, which frequently remain attached to the shell after the animal has been removed. The feeble hold of those muscles, even in a very recent state, is thus readily accounted for. Indeed, it is highly probable that the fixity of the body of Nautilus during the inhalation and forcible ejection of the respiratory currents is effected by the shell-muscles reacting upon one another, on the principle of a spring purchase, rather than by simple traction, as illustrated by the withdrawal of a gasteropod within its retreat, or the closure of the valves of a conchifer by the adductor muscles.

This view, which is supported by the foregoing facts, has its principal basis in the line of direction of the shell-muscles, and the angle at which they meet one another, at the root of the funnel-lobe; for the outer extremity of each being fixed, it follows that the first effect of the contraction of the muscular fibres would be to increase the angle just noticed; and this cannot possibly be accomplished, according to the recognized laws of muscular action, without tending to throw apart the points of origin, or in other words, exerting outward pressure against the internal wall of the shell, and thus, as it were, jamming the occupant tightly in its cell.

The action of the great lateral muscles of Nautilus here supposed, affords a remarkable contrast with the mode in which the posterior expanded arms of Argonauta embrace the exterior of its shell, particularly during the ejection of the expiratory current; while the withdrawal of the gasteropod into its abode, by the contraction of a veritable retractor, exhibits the exertion of muscular force in a very different direction.

In regard to the supposition that Nautilus macromphalus is the male of N. Pompilius, I may remark, that, besides my own specimen of the former, which proved to be a female, another, in very excellent condition, lately deposited in the Sydney Museum, is of the same

sex.

III. "Brief Description of a Ctenostomatous Polyzoon, allied to Vesicularia, occurring on the Australian Coast." By JOHN DENIS MACDONALD, Esq., Assistant Surgeon R.N. Communicated by Captain DENHAM, R.N., F.R.S. Received January 13, 1857.

In one of our visits to Moreton Bay, the sean was hauled at Moreton Island, and amongst the masses of sea-weed, &c. brought up with the net, I found numerous specimens of a very beautiful Polyzoon, a small portion of which I had previously dredged at Port Stevens from a depth of 5 or 6 fathoms.

The Polypidom may be said to have consisted mainly of rooted, spreading and plantlike portions, and short, straight creeping trunks, connected at both extremities with the fixed part of the former, so that the whole presented the appearance of an open lace-work, having all the transparency and lustre of glass.

The trunks and branches were nearly perfectly cylindrical, and composed of an outer membranous sheath distended with a clear fluid (which escaped with considerable force when the sheath was ruptured), and line-like reticulated vessels disposed in one plane, so as to communicate laterally with the polyp-cells, and divide the axis longitudinally into equal halves. The more central canals of this vascular plane combined to form a compound vessel, which opened into a spherical sinus with cellular parietes at the base of each branch.

The ramification of the Polypidom generally exhibited a trichotomous arrangement, with simple articulations occurring only where the branches were given off.

The cells were clustered in linear series on opposite sides of the branched axis, oval in shape, corneous in texture, with a terminal combed aperture, folding inwards by the contraction of four equidistant sets of muscular fibres, which imparted a quadrilateral figure to the opening.

The polypes were very minute, but exhibited distinctly all the important points of structure observable in those of Vesicularia and Bowerbankia, between which genera this polyzoon would appear to

lie. The ciliated tentacula, like those of Vesicularia, are eight in number, and do not possess the motionless hair-like processes which project from the back of each in Bowerbankia.

Although too much importance must not be attached to the actual number of tentacula surrounding the oral aperture, the tendency to multiply those organs must not be altogether forgotten. Thus, while there are but eight in Vesicularia, Bowerbankia densa and Bowerbankia repens possess respectively ten and twelve.

Both Bowerbankia and Vesicularia agree in the uniserial and unilateral distribution of the polypes, but in the present instance the cells are arranged in linear and bilateral clusters.

February 26, 1857.

The LORD WROTTESLEY, President, in the Chair.

The following communications were read :—

I. "Observations on the Natural Affinities and Classification of Gasteropoda." By JOHN DENIS MACDONALD, Assistant Surgeon R.N. Communicated by Captain DENHAM, R.N., F.R.S. Received January 13, 1857.

(Abstract.)

During his sojourn among the Feejee Islands, the author devoted much time to the anatomical investigation of recent Gasteropoda, with the view of discovering such indications of affinity in the details of structure as might serve as a basis for a natural arrangement of the order; and the present paper is designed to give a statement of some of the results of his researches, in order that the affinities of structure developed may be fairly examined and taken for what they are worth as principles of classification.

After pointing out objections to the foundation of primary divisions among the Gasteropoda on characters derived from the shell or from modifications of the respiratory organs, the author observes in respect of the value of sexual characters, that when the distinguishing features of a class are once satisfactorily determined, and this contains

forms in which the sexes are either separate, or combined in the individuals, no other characters can be of greater importance in establishing primary divisions. As a means of further subdivision according to natural affinities, he suggests distinctive characters derivable from the auditory sacs and concretions, and from the oral, lingual and gastric dental organs.

In Mollusca, as in Fishes, the auditory concretions present themselves in one of two forms, viz. solitary lapilli, usually named otolithes, or groups of small granules of a rounded oval or irregular shape, which have been designated by the term otoconia. The lingual teeth are either set together on a short and broad lingual membrane, and form what the author calls a lingual pavement, or on a narrow longitudinal band termed lingual ribbon or strap. The latter usually consists of a median rachis flanked by two lateral portions or pleuræ ; but in some cases the rachis, and in others the pleuræ are absent, and the number of longitudinal rows of teeth in these divisions may also differ in different genera. The fore part of the lingual membrane is supported by cartilage, so curved and fashioned as to receive the lingual sac behind and form a basis to the tongue itself projecting in front. This lingual cartilage may consist of a single piece thinned in the middle line, or of two or four distinct pieces, similarly arranged and wrapped together by muscle and ligament. The oral dental organs or labial plates are disposed either horizontally or laterally. In the former case a single plate may occupy the upper lip, or there may be two guarding the aperture of the mouth, and corresponding with both upper and lower lip, but the lateral plates are always in pairs. Gastric teeth occur in the Aplysiada and Bullidæ.

After pointing out further differences in the form and arrangement of the dental apparatus in different genera, the author thus describes the mode of development of the lingual teeth. "The lingual sac at first appears as a little cæcal process appended to the inferior part of the œsophagus, where it joins the oral cavity. In the median line of the floor of this sacculus, a few minute plates disposed in a longitudinal row form the rudiment of the future rachis, and the progress of their development may be distinctly traced on examining them, seriatim, from before backwards, in which direction, as their growth advances, they acquire a more perfect form. The internal row of pleural plates now makes its appearance, their development proceeding in a similar way; and after this follow the others according to their posi

tion, the more internal arising first. Thus the whole ribbon of dental organs increases in length and breadth by additions made respectively to its anterior extremity and sides; and each transverse row gradually moving backwards by the continued development and growth of others anterior to it, causes elongation of the lingual sac, which only attains its perfect state when these processes are at an end. The idea, therefore, that the new teeth are developed from behind forwards and successively brought into use, as in sharks and rays among fishes, does not appear to me to be correct."

In the annexed Table a rough arrangement is given of a considerable number of genera grouped together by the characters above referred to. Although the author thinks it improbable that any genera opposed to each other in those fundamental particulars can be intimately related, yet the facts are not advanced as the basis of a new elassification, but simply that they may yield their own weight, as so many available tests of affinity.