358 MOLLUSCA. 



passes along a similar canal or sinus in the shell, to enaoiC the animal to breathe without leaving 

 its shelter. There is also this distinction between the genera — that some want the operculum; 

 and the species vary in the filaments, fringes, and other ornaments that deck the head, the foot, 

 or cloak. 



We arrange these Mollusca under several families from the form of their shells, which 

 appears to be in sufficiently constant harmony with that of their respective animals. 



THE FIRST FAMILY OF THE PECTINIBRANCHIATA,— 



The Trochoides, — 



Is recognized by their shell having an entire aperture, without sinus or canal for a siphon, which the 

 animals have not*; and in being furnislied mth an operculum, or some organ as its substitute. 



The Trochusid.e {Trochus, Linn.).t 

 The month of the shell, angular at its exterior margin, approaches more or less to a quadrangular 

 figure, and is in an oblique plane in relation to the axis of the shell, because that part of the margin 

 next the spire advances more than the rest. The greater number of the animals have three filaments 

 on each side of the cloak, or at least some appendages to the sides of the foot. 



Among those which have no umbilicus, there are some in which the columella, in form of a concave arch, is 

 continuous, without any projections, with the exterior margin. It is the angle and advance of this margin that 

 distinguishes them from Turbo. These are the Tectai-ia, Montf. Several are flattened, with a sharp [spiny] margin, 

 whence they have been compared to the rowel of a spur ; these are the Calcar, Montf. Some again are a little 

 depressed, orbicular, glossy, with a semicircular aperture and aconvcx callous columella; Lamarck calls such Rotella. 

 Others have the columella marked near the base with a little prominence or vestige of a tooth, similar to that of 

 Monodonta, from which these Trochoides differ only in the general shape of the aperture, which is, in the present 

 instances, a little deeper than wide :— they are the Cantharides, Montf. The aperture in others is, on the contrary, 

 much wider than deep, and their concave base gives them a resemblance to the Calyptreae; these Montfort names 

 Entonnoirs. Others, in which the aperture has the same great proportional width, have the columella in the form 

 of a spiral canal. And those which have the shell turreted {Telescopium, Montf.) resemble the Cerithia. 



Among the umbilicated Trochusidae, some have no longer any projection on the columella ; the greater number 

 are flattened, and have theexterior angle sharp. Of this kind is Trochus agglutinans, Linu., remarkable for its habit 

 of gluing and incorporating with its shell, in proportion as it grows, different foreign bodies, such as gravel, frag- 

 ments of other shells, &c. It often covers its umbilicus with a testaceous plate. There are some also with rounded 

 margins, of which we have a common example on our coasts, (Tr. einerarius, Linn.). Other umbilicated Trochi 

 have a prominence near the base of the columella : and lastly, in others it is crenulated throughout its length. 



The Sulariiim, Lam., is distinguished from the other Trochi by its obtusely conical spire, whose broad base is 

 perforated with a wide and deep umbilicus, in which the eye can trace the margins of all the whorls winding up 

 [like an elegant miniature staircase], and prettily crenulated. The Euomphalus, Sowerby, are fossil shells similar 

 to Solarium, but without crenulations on the inner whorls of the umbilicus. 



The Periwinkles (Turbo, Linn.) — 

 Comprise all the species with the shell perfectly and regularly turbinate, and of which the aperture is 

 quite round. From a detailed examination of them, they have been greatly subdivided into genera. 

 The Turbo, Lam., properly so called, have a round or oval thick shell, with an aperture completed on 

 the side of the spire by the penultimate whorl. The animal has two long tentacula ; the eyes raised 

 on [short] pedicles at the exterior liase ; and, upon the sides of the foot, membranous expansions, 

 either simple or fringed, or furnished with one or two filaments. To some of them those stony thick 

 opercula belong which may be frequently observed in collections, and which were formerly used in 

 medicine under the name of Unguis odoratus. Some are umbilicated {Meleagris, Montf.), and some 

 are not so {Turbo, Montf). 



The Delph'mida is a shell as thick [and soliil] as the Turbo, but subdiscoid, and its aperture is entirely formed 

 by the last whorl, and without a varix. The animal resembles the Turbo. The common species (Turbo dclpltinux, 

 Linu.) takes its name from the branched curved spines that arm the whorls, and which have given rise to a com- 

 parison of it to a dried fish. 



The Pleurotoma, Defrance, are fossil shells with a round mouth, and a narrow deep incision on the outer margin. 

 It is probable that this incision corresponds, as in Siliquaria, with some fissure of the cloak. M. Deshayes reckons 

 already more than twenty-five fossil species. The Scissurelhe of M. d'Orbigny are recent species. 



The Turritella, Lam., have the aperture of Turbo, but the shell is thin and elevated into an obelisk, or turreted. 



• Hence Blainville denominates the order A^iphonobrarichiitii. t Family IJojriostomuta of Dc Blaiiiville. 



