58 ACHATINA. 



a flat surface, in an open situation, the base will be short and flat; 

 if it be placed in a hollow among other growing substances, it 

 will be lengthened out in order that the aperture of the shell may 

 be even with the outer surface of the surrounding mass ; and if, 

 as in the Acastse, it be imbedded in a soft and loose substance, 

 the base, being left to itself, will take a regular form. The 

 Acastse are found imbedded in sponges. Ex. Balanus Montagui, 

 of Great Britain, fig. 26. Also found in the Pacific ocean and 

 Philippines. 



ACAVUS. Montfort. Fam. Limacinea, Blainville ; Colimacea, 

 Lamarck. A division of the genus Helix, which may be con- 

 sidered synonymous with De Ferrusac's sub-genus Helicogena. 

 De Montfort has given H. Hsemastoma, as an example. Fig. 267. 



ACCESSARY VALVES, are the smaller or less important testaceous 

 plates, found on the hinge or dorsal margins of the true valves of 

 some shells. Example, the small plates on the hinge of Pholas, 

 fig. 55, a. The Pholades were placed by Linnseus and Bruguiere 

 among multivalve shells. 



ACEPHALOPHORA. Blainville. («, without ; ^aXe, head.) 

 The third class of the type Malacozoaria, Bl. including all bivalve 

 shells, the animals of which have no distinct head. This class 

 corresponds with the Conchifera of Lamarck, and is divided into 

 the orders Palliobranchiata, Rudistes, Lamellibranchiata, and 

 Heterobranchiata, the last of which contains no genera of testa- 

 ceous Mollusca. 



ACHATINA, Auctorum. Fam. Colimacea, Lam. (from Achates, 

 an agate.) Fam. Limacinese, Bl. Gen. Polyphemus, Montf. — 

 Bescr. Shell oval or oblong, sub-turrited, light, thin ; aperture 

 oval, or pyriform ; outer lip sharp ; columella smooth, tortuous, 

 truncated, so as to form a notch at its union with the outer lip. 

 — Obs. It is from this notch that we are enabled to distinguish 

 Achatinse from Bulini, which, moreover, generally have a reflected 

 outer lip. The Polyphemi of Montfort have an undulation in 

 the centre of the outer lip. Achatina Virginea, fig. 286. Poly- 

 phemus Glans, fig 288. These land shells are found in various 



