Silicaria. A tubular shell, spirally con voluted, and laterally divided through its whole length by a narrow fissure ; the mouth roundish. Linnaus himself consi ders it as doubtful whether the shell which forms this genus (Serpula anguina) should he placed under the genus Ser pula, or not ; but Brugulere, Lamarck, and Daudin, have thought it necessary to establish for it this new genus.
Penicellus. A long tubular conical shell, the superior extremity closed by a disk, beset with numerous short tubes, and sur rounded by a projecting coronet ; the infe rior extremity having been fixed to some solid body. Linnaus had doubted whether the shell, of which Brugulere has formed this genus, ought not to be placed under the genus Teredo, rather than under that of Serpula, where, however, he left it. Nautilus. A spiral and nearly discoidal shell, the last turn of which envelopes the rest, and the sides of which are smooth : numerous chambers, formed by transverse smooth septa, perforated by a tube. This genus is divided by Lamarck into three. 1. Nautilus, 2. Spirula, 3. Orthocera.
Orbulites. A spiral, and nearly discoidal shell, the last spire enveloping the rest ; the sides articulated by winding sutures; the transverse septa being pierced by a marginal tube. The shells which La marck has assumed for this genus have been hitherto confounded with the Am monites; they are only known as fossils and as casts.
Ammonites. A discoidal spiral shell, the turns contiguous and all visible, the sides articulated by foliated sutures, and the transverse septa terminating in wind ing processes, and pierced by a marginal tube. These, like the shells of the pre ceding genus, are only with certainty known to exist as fossils.
Planulites. A discoidal spiral shell, with contiguous and visible turns, with smooth aides, and transverse septa, not foliated. These shells are supposed to bear the same relationship to the Ammonites as the Nautili do to the but the difference between them and the Ammo nites is so little as must often be with dif ficulty discoverable.
Camerina. A shell with a single valve, without any external spire, the internal part divided into numerous chambers by imperforated septa. This genus was form ed by Brugutere, for the reception of the fossil shells long known as Lapides lenti culares and Nummulites.
Rotalites. A depressed orbicular,discoi dal chambered shell, smooth on the upper, radiated on the under side, with tubercu lar and unequal points in the centre ; the edge carinated, and having a small trigon al marginal opening. The fossil shell on which this genus was formed is figured and described by Guettard in his Memoirs on Fossils.
Turrilites. A spiral turbinated shell, with contiguous and visible turns, the sides articulated by winding sutures ; transverse pierced septa, with foliated ter minations; the opening round. The shells belonging to this genus are all fossil, and even the fragments of their casts are very rare. The characters of the shells of this genus are very striking, their internal structure being similar to that of the Am monites, while they possess a turriculated or wreathed form.
Baoulites. A straight cylindrical and rather conical shell ; the sides articulated by winding sutures ; the transverse septa terminating in foliaceous forms. This ge nus is founded on a fossil shell, described by Faujas St. Fond in his Natural History of St. Peter's Mountain, near Maestrecht, and which he considers as a straight Am. monite.
Spirula. A shell partially or complete ly in a discoidal spire ; the turns separate, and the last extending into a straight line ; transverse smooth septa, pierced by a tube; the opening round. This genus comprises also the Lituites, a fossil shell, bearing a very close resemblance, in form, to the recent shells of this genus, but very far exceeding them in size.
Belemni tes, a many chambered, straight, long, conical pointed shell, full at its summit, and possessing a lateral cleft.— These are only known in a mineral state.
Sam., in chemistry. Shells of marine animals, and of all eggs, consist chiefly of carbonate of lime, and yield a very pure lime, for which they are used on the sea coast. They afford, likewise, a small portion of phosphate of lime. There are two great classes of marine shells, the testaceous and crustaceous, the former are by much the most numerous, and in clude all the marine shell animals that have not legs, and the power of trans porting themselves from place to place. The crustaceous shells are those of the lobster, crab, prawn, &c. who carry their shell as a protection from external injury, and not as a place of residence. Mr. Hatchet has made many experiments shells, and he observes that marine shells, for the most part, are either of a porcella neous aspect, with an enamelled surface and fibrous texture, or they are composed of the substance called nacre, or mother of pearl. The first kind dissolved in acids with strong effervescence; and their solu tions afforded no trace of the phosphate of lime: they contained only carbonate of lime, and the animal matter, which acts as a cement to this, and which he supposes to be albumen in various states of indu ration ; this is in small proportion ; hence shells exposed to heat exhale but little empyreumatic animal odour ; they emit no smoke, and when dissolved in acids no vestige of it could be discovered. In shells of the other description approach ing to nacre, the earthy matter is carbon ate of lime, but in a smaller proportion, while the animal matter is in considera ble quantity. These give out smoke and an empyreumatic odour, when exposed to heat ; and when acted upon by acids, give out less carbonic acid gas, and leave a large quantity of a membranaceous or cartilaginous residuum. This substance often constitutes a large part of the shell, as in that of the oyster or muscle, and is so much indurated as to be no longer ge latinous ; and in all shells of this division it appears to be deposited in layers, each having a corresponding coat of carbonate of lime.