360 Div. 2. MOLLUSCA.— GASTEROPODES. Class 3. 



the madrepore ; and, increasing itself in size and length as the madrepore increases around it, it keeps 

 the aperture even with the outer surface of the coral, and thus grows, in some instances, to a consi- 

 derable length. This singular testaceous parasite is common in the coral rocks of the Isle of France, 

 and its tube sometimes reaches the length of three feet.] 



SiLiauARiA, Brug. — 

 Resembles Vermetus in the head, the position of the operculum, and in the tubular and irregular shell ; 

 but there is a fissure on the whole length of the shell which follows its contour, and which corres- 

 ponds with a similar cleft in that part of the cloak which covers the branchial cavity. Along the 

 •whole side of this cleft is a branchial comb, composed of numerous delicate and tubular-like leaflets. 

 Tiinmieus left these shells also in Serpula ; and until a very recent date they were believed to be mem- 

 bers of the class Annelides. [The remarkable operculum is similar to the pod of a INIedicago, consisting 

 of a spiral lamella rolled five times round an axis like a pulley. This horny lamella is very lustrous 

 underneath, farinaceous or subpubescent above, and subcrenate on the under side of the rim, 

 with short striolre. It is convex in the centre, and the projectioi; is multilocular, very exactly resem- 

 bling a Cristellaria or RobuUaa.'] 



THE EIGHTH ORDER OF THE GASTEROPODES. 



THE SCUTIBRAXCHIATA.* 



The order comprises a certain number of Gasteropods having a consiilerable resemblance to 

 the Pcctinibranchiata in the form and position of the branchiae, as well as in the general form 

 of the body, but they are complete hermaphrodites. Their shells arc very open, without an 

 operculum, and the greater number are not in any degree spiral, so that they cover their 

 animals, and particularly the branchia;, in the manner of a shield. Tlie heart is traversed by 

 the rectum, and receives the blood by the two auricles, as in the majority of the Bivalves. 



The Haliotides {Haliotis, Linn.) — 

 Are the only family of this order in which tlie shell is turbinated ; and from those shells it is distin- 

 guished by the excessive amplitude of the aperture, and the flatness and smallness of the spire, which 

 is seen from within. This form has caused it to be compai'ed to the ear of a quadruped. 



In the Ilalioiis, Lam., the shell is perforated along the side of the columella with a series of holes ; anil when 

 the last hole remains incomplete, the shell has the appearance of bein?^ eniari;inate. The snail is one of the most 

 richly adorned of Gastcrojiods. A doiil)lc membrane, with a fiirbelowed margin, and fnrnishod with a double row 

 of fdaments, extends, at least in the commonest species, round the foot, and on to the month : outside its loiigf 

 tentacula are two cylindrical pedicles, which support the eyes. The cloak is deeply cleft on the right side, and the 

 water, which passes through the holes of the shell, gains access, by the medium of the cleft, to the branchial cavity. 

 Along the margins of the cleft there are also three or four filaments, wliich the animal can also protrude through 

 the holes of the shell. The mouth is a short proboscis. 



I'atlu/la, Moiitf. [S/oma/el/a, Lam.] has an almost circular shell ; almost all the liolcs obliterated ; and a deep 

 groove that follows the middle of the whorls, and shows itself exteriorly by a corresponding ridge. 



Utomalifi, Lam., have a more concave shell, with a more prominent spire, and without holes : tliey otherwise 

 resemble the Haliotis,and connect that genus with certain kinds of Turbo. The animal is less adorned than Ilaliotis.f 



The following genera, dismembered from Patella, have the shell quite symmetrical, as well as the posi- 

 tion of the heart and branchiae. 



FisstTRELi.A, Lam. — • 

 Have a broad, fleshy disk under the belly, as the Patella ; a conical shell placed over the middle of the 

 back, but not covering it comj)lctc]y, and perforated in the sununit with a small aperture, which serves 

 botli for the passage of the excrements, and of tlu^ water necessary to respiration : that aperture pene- 

 trates into the cavity of the branchirc situate over the front of the back, at the bottom of which the anus 

 oi)ens ; and this cavity is moreover widely patulous over the head. Tlicre is a branchial comb on each 



• I\I. de nlaiiiville unites this ftnd the folluxviii); order in his sub- 

 class t*tirin:rfili(i!ttfihttrii hrrm':phri/dit(l. 



■r yP'idiil.i iiEid St'imnthi (lli;it cnnstitute but cue ijcnu^, accortiinn 

 li> bowcrby,) Hic piared iti ihe order Pcetiiiibrancliiiitii tty Kiiijf, where 

 we fiiirl uUo next llicni the f'clutina uf Kleinnijiig, distinguished by its 



neiitoid thin shell with ft wide ent're aperture, without an opi^rciihini 

 His Sti/I'nia t^Sti/liJer, Broderip) 1ms aI-M» no opereuluin, but tlic spire 

 is pointed and aeute. One species lives on tl»e Jiehi.pjs ; another im- 

 beds itself in Star/ish "^ 



