3G8 MOLLUSCA. 



the madrepore ; ami, inrroaslng; itself in size ami length as the madrepore increases armmd it, it k^eps 

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

 derable length. Tliis singidar testaceous parasite is common in the coral rocks of the Isle uf France, 

 and it« tuhe sometimes reaches the length of three feet.] 



SiLiauARiA, Brug. — 

 ResenihlesVermetns in the head, the position of the operculum, and in the tubular and irregidar shell; 

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

 pomls 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 eoml>, coni|Mt>ed uf numerous delicate and tul)iilar-!ike leaflets. 

 LinuEcus left these shells also in Serpnla ; and until a very recent date tliey were believed to be mem- 

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

 of a spiral lamella rolled five times round an a\i^ like a pulley. This huniv lamella is very lustrous 

 underneath, farinaceous or suhpubi-scent above, and sidicrenate on the under side of the rim, 

 with short striokc. It is emivex in the centre, and the projection is multilocular, very exactly resem- 

 Liling a CrisieUaria or Robulina.~\ 



THE EIGHTH ORDER OF THE GASTEROPODES. 



THE SCUTIBRANCHIATA.* 



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

 the Pectinibranchiata in the form and position of the branchix% as well as in the general form 

 of the body, but they are complete liermapbrodites. Their shells arc very open, ^\itbout an 

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

 animals, and partictdarly the branchiae, in the manner of a shield. The heart is traversed by 

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



THii IIaliotides {[Inliolis, Liiiii.) — 

 iVre the only family of this order in \\liieh the :>hcll is turbinated ; and from those shells it is distin- 

 guished by the excessive amplitude of the aperture, and the IbUness aad smallaess of the spire, whieh 

 is seen from within. This form has caused it to he compared to the ear of a quadruped. 



In the }{filio/is; Lam., the sbell is perforated along' the side of the columella with a series of holes ; and when 

 the last bole remains incomplete, the sheh has the appearance of hein^" eniarginate. The snail is one of the most 

 richly adorned of Gasteropods. A double niembrane, with a furbelowed margin, and furnished with a double row 

 of hlaments, extends, at least in the commonest species, round the foot, and on to the month : outside its hnvj; 

 tentacula are two cylindrical pedicles, which support the eyes. I'he cloak is deeply cleft on the ri2;ht side, and the 

 water, which passes through the lioles of t lie shell, g;ains access, by the mediuuj of the cleft, to the branchial cavity. 

 Alono;^ the margins of the cleft there are also three or four filaments, which the animal can also protrude throug^h 

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



PadoUa, Moiitf. [Sfomah'Ild, Lam.] has an almost circular shell ; almost all the holes iiMiteratcd ; and a deep 

 groove that follows the middle of the whorls, and sliows itself exteriorly by a eon'espnuding ridge. 



•^tomalio. Lam., have a more concave shell, with a more prominent spire, and without holes : they otherwise 

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



The following general dismeiribere'd fr^uu Patella, have tlie shell quite symuicti"ie;d, as well as the posi- 

 tion of the heart and branehi;e. 



FissuRELf.A, Lam. — 

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



