I. HALYOTIS. The shells of the animal of this genus have frequently been described, but it is to Cuvier that we owe the most accurate details concerning the struc ture of their inhabitants. The foot is oval and large. The sides of the body all round are ornamented with one or more rows of simple or branched filaments. The shell is placed on the back, with the spiral part behind, and the row of holes on the left side, through which some of the filaments are protruded. The animal is at tached to the shell by a single large muscle. The entry to the branchial cavity, which likewise contains the ter mination of the rectum and oviduct, is on the back. The gills are in two ridges, consisting of complicated branched filaments. At the entrance of the cavity, the cloak is furnished with a slit, the left margin of which rests upon the pillar of the shell. The edges of this slit are furnished with filaments, which pass through the anterior holes of the shell. The use of this singular ar rangement is unknown. The branchial cavity likewise contains the viscous organ, in common with the Pectini bra nc him.
The hood is emarginate, with a long tentaculum on each side, behind which, towards the side, is a cylindri cal protuberance, bearing the eye at the top. The mouth is in the form of a short proboscis, with two corneous plates as cheeks, and a long narrow tongue extending backwards, and covered with spines. The pharynx is dilatable, with internal folds. The salivary glands are very small. The gullet is very short. The stomach is divided into two portions. The first is striated longitu dinally with a glandular structure, and receives a biliary duct. The second is separated from the former by a valve, is smaller, with transverse stria?, and a double ridge. It likewise receives bile through two apertures. There is another valve at the pylorus ; and the intestine, after making some turns, is surrounded by the heart. There is an auricle on each side, receiving the aerated blood from each of the gills.
The species of this genus are by no means numerous. Montfort separated those in which the marginal holes were nearly obliterated, with an internal groove and external ridge in the line of their direction, and formed them into a genus which he terms Padolles. Lamarck
likewise separated those whose shells are destitute of the marginal holes, under the generic appellation Sto matia.
2. CitruLus. This genus is represented by the Pa tella hungarica. The foot is complicated on its ante rior margin. The shell adheres to the animal by a cir cular muscle, leaving an opening in front, for the issue of the head and entrance to the branchial cavity. The gills form a single ridge across the roof. The mouth is in the form of an extended proboscis, with a deep groove above. The tentacula, which are two in number, have the eyes at the external base. The anus is on the right side of the branchial cavity.
We possess two species of this genus in our seas, Hungarica and Antiquata, to which M. Cu vier is disposed to add the Bulb velutina of Muller. This last shell, however, we have already noticed as belonging to the Pectinibranchix.
3. FISSURELLA.—The animals of this genus have the foot like the preceding, and the back is proteeted by a conical shell perforated at the apex. The shell is united to the cloak by a circular muscle, open in front. The cloak forms a duplicature in front for the branchial ca vity, which extends to the perforated apex of the shell. The gills consist of two ridges ; at the dorsal extremity of which is the anus. It is probable that the excrements are ejected at the perforation in the apex of the shell, and likewise the water which enters the branchial cavity in front. The head is furnished with two tentacula, bear ing the eyes at the external base.
The Patella grxca and apertura may be quoted as British examples of the genus.
4. EMARGINULA.—This genus differs from the former in the apex of the shell not being perforated. Its place, however, is supplied by a slit on the anterior margin, the entrance to the branchim and anus. The foot is surrounded with a row of filaments, and the eyes are supported on short foot-stalks, characters in which it ap proaches the genus Halyotis. The Patella fissura of conchologists is considered as the type of the genus.