attention to its very long and curious crystalline stilette, unconnected with the minute foot. With respect to that _quæstio vexata_, the sexes of the Lamellibranchiata, he observes that any number of individuals of _Cyclas_ may be examined, and young fry will be found in the branchial laminæ in all; that all Oysters have ova, and also all individuals of _Pecten maximus_, the subpedal mass being visibly composed of an ovary and a testis. He is obliged to believe that one species of British _Anodon_ is universally oviferous. But the common Edible Cockle appears to have the individuals of different sexes, and the same may be said with regard to _Mytilus edulis_ and _Patella_. The spermatozoa in the Cockle are oblong and a little curved, and torulated, as it were, whilst they are pear-shaped in _Mytilus_; they are also extremely minute, and their appendages must be very fine, for with a power magnifying 500 diameters they are scarcely to be seen. In the shell of a _Patella_, _Emarginula_, or _Haliotis_, we have the two conjoined valves of a lamellibranchiate mollusk; and through such forms as _Calyptræa_, _Hipponyx_, _Navicella_, and _Nerita_, we arrive at the ordinary form of the gasteropod with its operculum. Then follows a disquisition on the progressive tendency to a spiral geometry in these animals, due to a varying plan of conformation, and not to the force of the heart, there being generally an atrophy of the left side of the body. In _Nautilus_ and _Argonauta_, the shell and mantle are reversed in position to what they are in the Gasteropods, whilst _Sepia_ and _Hyalæa_ agree rather with the latter. The symmetrical shell of the lower Gasteropods undergoes a lateral torsion in the higher, spiral forms, to become again symmetrical in the Cephalopoda. The branchiæ in _Patella_ retain a position analogous to that of the same organs in the Lamellibranchiata; in some Chitons they have a tendency to retract towards the anus, as in _Doris_; in _Fissurella_ they waste at the sides and become developed above the neck, as in the spiral Gasteropods; but in them, the right branchia, and right side of the mantle are principally developed. From this torsion arises the form and spire of the shell. In _Aplysia_, where the branchial fissure is far back and to the right side, the right respiratory nerve preserves a superior position, and passes backwards to form its ganglion at the front of the branchial opening; the left, on the contrary, passes under the oesophagus to form a second ganglion, not mentioned by Cuvier, behind the first. In the more spiral Gasteropod the torsion is greater; the right nerve, for instance, mounts upwards over the digestive canal to form its ganglion quite in the left flank, whilst the left goes below the digestive canal to attain the right flank. In _Sepia_ the branchiæ are again symmetrical and abdominal.