35S MOLLIJSCA. 



passes along a similar canal or sinus in the shell, to enable the animal to breathe "witliout leaving 

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

 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 iu sufficiently constant harmony with that of their respective animals. 



THE FIRST FAMILY OF THE PECTINIBIUXCIIIATA,— 



The Trociioidks, — 



Is recognized by their shell having; an einirc aperture, witliont siinis or canal for a :^i]tlion, which the 

 animals have not*; and in Ijcnij furnished witii an operculum, or some organ as its stiljstitute. 



The Trochusid.t^ (Trocltua, Linn.).t 

 The mouth of the shell, anjuhir at its cxterinr margin, approaches more or less to a quarlranenlar 

 figure, and is in an oblique plane in relation to tln_- a\is of tbe shell, because that jiart of tlie margin 

 next the spire aiivanees more than tbe rest. Tbe greater nundjer of tbe animals bave tliree iilaments 

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



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

 continuous, without anj' projections, with the exterior marp^in. It is tlie an;jle and advance of this maryin that 

 distin*!^uishes them from Turbo. These are the Tectaria, Montf. Several are flattened, with a sharp [spiny] margin, 

 whence they have heen compared to the rowel of a spur ; these are the Calcar, IMontf. Some ag"ain are a little 

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

 (Hliers have the columelia marked near the base with a \\\.{\\^ pi-nmineiice or vestige of a tooth, similar to that of 

 Monodonta, from which these Trochoides difter only in tlir ^i.-ncral shape of the aperture, which is, in the present 

 instances, a little deeper than wide:— they are tlic Canflmrhh-s, .■\In)ilf. The aperture in others is, on the contrary, 

 much \vi(ii.-r' than deep, and their concave base gives them a resemblance to the Calyptrese; these I\Iontfort names 

 Enfoiuiiiir.w < Jthers, in which the aperture has the same p:reat proportional width, have the columella in the form 

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



Among the umbilicated Trochusidc-e, some have no long-er any projection on the columella ; the greater nuraher 

 are flattened, andhave theexterior angle sharp. Of this kind is Trochus aaghidnanN, Linn,, remarkable for its habit 

 of gluing and incorporating with its shell, in proportion as it grows, di tie rent fu it i:;ii b[idn.-s, such as gravel, frag- 

 ments of other shells, &c. It often covers its umbilicus with a testaceous plate. 'Hii'H' an- some also with rounded 

 margins, of which we have a common example on our coasts, {Tr. r'nicrariu.s-, Linn.i. f )|licr nmbibcated Troclii 

 liave a prominence near the base of the columella : and lastly, in others it is crcunhiti.'d llinin^hnut its leii;_'-th. 



The Sulariiiiii, Lam., is disting-uished from the other Trochi by its ubtusely conical sjurc, uliose broad base is 

 perforated with a wide and deep umbilicus, in which the eye can trace the mai-gins of all the whorls windins; up 

 [like an e!e2:ant miniature staircase], and pi-ettily crenulated. The EuomphalKS, Sowcrby, are fossil t-hells similar 

 to Solai'ium, but without creiiulations on the inner whorls of the undnlicus. 



The Periwinkles {Turbo, Linn.) — 

 Comprise all tbe species with the shell perfectly and regularly turbinate, and nf which the apcrtni-e is 

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

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

 tbe side of the spire by the penultimate v^liorl. The animal lias two long tentacula ; the eyes raised 

 on [short] pedicles at the exterior base ; and, upun tbe sides of the foot, mendjrauous expansions, 

 either simple nr iVingeib or furnished with u)ie ur iwu iilaments. To siuiie "i them ilnt^e stmiv thick 

 opercula belong wliich may be frequeutly ol)ser\ed in culleetitms, and wliieli were bu-merly used iu 

 medicine under tbe name of Uugn.h odoratus. Some are umbilicated {Mc/caf/r/s, Mniilf.), and some 

 are not so {Ti'rOo, iVIontf.). 



'jhc lirlphiiniln is a shell as tliick [and solid] as thr Turbo, but sulidiscdul, ;nid its a[)rrture is entirely formed 

 by (he last uliorl, and without a varix. The animal rrsemhles tbe Turiio. Tin.- eiMnnnm species (Tiirho (hiph'ninx, 

 Linn.) takes its name from the branched cur\ed spines tliat arm the w hints, and wliieh lia\'e yiven rise to a cum- 

 ]i,irison of it to a dried (ish. 



The I'h'in-ohniHi, 1 )i-fraiice, are fossil shells with a round mnntb, and a narrow divinncision mi the outer margin. 

 li is prnliablr tha.t this incision corresponds, as in Silir|uaria, witli smm- fissure of iheehiak. M. Doshayes reckons 

 iih-'-udy mure tli;ni t\\enly-(ive fossil species. The Scissnrrll.r nf M. d'Orbigny are recent species. 



'l]ii- 'i'urnlrlhi, l,;inK, hn\e the aperture of Turbo, but thi' slull is Lhm and elevated into an obelisk, or turreted. 



• Hi'iLi.-e IMiiiiivitIc ilcD iriiitL-, tlj^ onkT -hii>honobi-'Uu:hiufa. ^ Fii.Liily Gviiii.'it.,,,!,//.! <,{ I),- BUCiuvWi,:. 



