CHISMOBRANCIIIA'TA, an order of Mollusea, forming, in De Blainville's system, the second order of his second sub-class, Paracephalophora Monaca. The following is his definition of the order :—Organs of respiration aquatic, brat:chi:11 or pectinated, situated at the anterior part of the back, In A large cavity communi cating with the ambient fluid by a wide oblique anterior alit. Mouth toothless, but provided with a long lingual riband-like organ. Shell either none, or internal, or external, very much depressed, with a very large entire aperture, and without any pillar (columella).
This definition In Incorrect, in so far as it states that in eome instances there in no shell ; for Coriocella, the only genus described by De Blairsville as being without any alto% has a horny one, AR Cuvier observes, though it is very delicate and flexible and nearly membranous. Curler, who places three of the genera, Sigarans, Corioeella, and Cryptostonut, under his Capuloitles, a family of his order Gasteropoda pectinibranehiata, observes that De Blainville places the greater part of the Capuloida under his non-symmetrical Hermaphrodite paracepha/ophora, or Calyptracians ; but that they appear to him (Cuvier) to be all dicecious.
The geographical distribution of this order, which, according to De Blainville, is marine and probably herbivorous, is wide.
Coriocella.—Body elliptical, very much depressed, having the borders of the mantle very delicate, notched in front, and spreading out very largely on all aides. Foot oval, very smalL Head scarcely distinct ; two tentacula hidden under the shield of some size, but short and contractile. Eyes at the external base of the tentacula. Back some what rounded, and, ac cording to De Blainville but this as we have already seen is an error—with out any shell, external or internal.
C nigra, Blain ville. The only species of the genus, and described by De Blainville from a specimen in his collection.
Locality, seas of Mauritius. Cuvier places this and the two following genera under his Gasteropoda peetinibranchiata.
Sigaretus. —Shell more or less thick, flattened, with an ample and round aperture and but little Spire, the whorls of which increase very suddenly; and enveloped during life in a spongy shield, which con siderably encompasses its borders as well as the foot, and which is the true mantle. In front of this mantle there is a notch and a demi canal, which serve to conduct the water into the branchial cavity. The tentacula are conical, and the eyes are placed at their external base. The male organ, according to Cuvier, is very large.
De Blainville thus subdivides the genus:— a. Species with a very delicate and smooth shelL
Example, S. construe.
b. Species with a thick and solid shelL Example, S. haliotoideus.
The species of Sigaretus have been found at depths varying from 5 to 15 fathoms on sandy bottoms.
Fossil Sigareti.
Defrance enumerates three fossil species, one from the Plaisantin, one from Grignon, and another from the environs of Bordeaux. G. B. Sowerby says that the fossil species are few and rare, and that they occur in the London Clay at Barton, and in the contemporaneous formations in France and Italy. The species in the Calcaire Orossier at Grignon, he adds, has a small umbilicus. Deshayes in his 'Tables' gives eleven living species and four fossil (tertiary) ; one, S. depresses, living in the seas of the Molucca Iolanda. The fossils occur in the Pliocene, Miocene, and Eocene periods of Lyell. S. e.rcavatue is found in the Crag.
Cryptostoma.—Shell very like that of Sigaretus, carried with the head and abdomen, which it covers, upon a foot four times its size, cut almost squarely behind, and which produces anteriorly a fleshy and oblong part, which makes nearly one-half of the mass. The animal itself has a flat head, two tentacula, and a large pectinated branchia on the plafond of its dorsal cavity. The male organ is placed under the right tentaculum.
Example, a Leachii.
O.ryn6e.—Body gasteropod, with a large dorsal shell, anterior, bulli form, and with a simple spire. Foot narrow. Branchia3 marginal, striated transversely. Mantle widened into two lateral wings. Ten tacula two, not retractile.
Example, 0. oliracea.
Velutina.—Animal oval, sufficiently protuberant (boinb6), hardly spiral; border of the mantle simple anteriorly, and double for the whole of its circumference ; the internal lip thickest and tentacular. Foot thick. Tcntacula large, obconical, distant, with a small frontal veil between them. Eyes black, smile at the external side of the base of the tentacula. Mouth large, at the extremity of a sort of muzzle. Respiratory cavity large, without any trace of a tube, and containing two unequal pectinated branchiss ; orifice of the ovary at the base of the male organ, situated at the root of the right tents cilium. Muscular attachment of a horse-shoe shape, very slight behind and open before. Shell external with an epidermis, patelli form, with a small lateral spire, and without a columella. Aperture large, the edges almost continuous, and sharp : the right border united to the left by a lamellar calcareous deposit.
Example, V. capuloidea (Helix la-vigata. Linn.). [VEturixin5.]