1037 



CHISMOBRANCHIATA. 



CHITONID.E. 



103S 





Hermaphrodite paracephalophora, or Calyptracians ; but that they 

 appear to him (Cuvier) to be all dioecious. 



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 

 sides. 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.niyra, Blainville. The 

 only species of the genus, 

 and described by De Blainville from a specimen in his collection. 

 Locality, seaa of Mauritius. Cuvier places this and the two 

 following genera under his Giuteropoda, pectinibranckiata. 



Siyaretu*. 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. convejctu. 



Corioeella nigra. 





Siyaretui contents, teen from below. 



Sigaretiu mmtxul, idc view. 



i. Species with a thick and solid shell. 

 Example, S. haliotoideus. 



Sigarttut haliotoidevt. 



The species of Siyarttui have been found at depths varying from 

 & to 15 fathoms on sandy bottoms. 



Potsil Sigareti. 



Dcfrance 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 Grossier 

 at Grignon, he adds, has a small umbilicus. Deshayes in his ' Tables ' 

 gives eleven living species and four fossil (tertiary) ; one, S. depreeu, 

 living in the seas of the Molucca Islands. The fossils occur in the 

 Pliocene, Miocene, and Eocene periods of Lyell. S. excavdtut is found 

 in the Crag. 



Cryptottoma. Shell very like that of Sigaretui, 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, C. Leachii. 



Oryptostoma Leachii. 



O.rynSe. Body gasteropod, with a large dorsal shell, anterior, bulli- 

 form, and with a simple spire. Foot narrow. Branchia; marginal, 

 striated transversely. Mantle widened into two lateral wings. Ten- 

 tacula two, not retractile. 



Example, 0. olivacea. 



Velutina. Animal oval, sufficiently protuberant (bombd), hardly 

 spiral ; border of the mantle simple anteriorly, and double for the 

 whole of its circumference ; the internal lip thickest and tentacular. 

 Foot thick. Tentacula large, obcouical, distant, with a small frontal 

 veil between them. Eyes black, sessile 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 branchia; ; orifice of the ovary at 

 the base of the male organ, situated at the root of the right tenta- 

 culum. 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 lieviyata. Linn.). [VELUTiuiD.fi.] 



I~i lutiiia capuloidea, 



CHITON. [CHITONID.E.] 



CHITONELLUS. [CHITONIDJS.] 



CHITONID^, a natural family of Gasteropodous Mo'dmca, 

 affording the only known instance of a protecting shell formed of 

 many portions, or, as they have been somewhat incorrectly termed, 

 valves, often in contact and overlapping each other, but never truly 

 articulated. The following cut will give some idea of the structure 

 of this shelly covering. 



These plates are bound together by a coriaceous border, which, 

 as we shall presently see, is either plain or beset with bristles, 

 spines, &c. 



The early naturalists took these shells for the peculiar armour of 

 certain serpents, a conclusion to which they were doubtless helped 

 by the love of the marvellous, so strongly shown in the accounts of 

 the older travellers. By degrees the true condition of these mollusks 

 became better known ; and the opposite opinions of Linnaeus and 

 Adanson divided the naturalists of their age. The former arranged 

 these shells among his Multivalves, a class entirely artificial, and like 

 all artificial classifications comprising the most heterogeneous forms. 

 Adanson, on the contrary, took nature for his guide, and carefully 

 observing the animal itself, while he regarded the shell as of compara- 

 tively small importance, placed Patella and Chiton side by side in his 

 method. But the Linnasan school long reigned paramount ; and 

 Adanson's labours were comparatively forgotten, when Cuvier began 

 to reform the crude state in which he found the Mollusca, and 

 Lamarck and others aided in the work. Cuvier, who made anato- 

 mical investigation the basis of his opinions, at once pronounced in 

 favour of Adanson. Lamarck afterwards adopted the same conclusion, 

 but not till he had previously placed the Chitons at the end of the 



