58 ACHATIlSrA. 



AC AST A, Leach. Order, Sessile Cirripedes, Lamarck. Balanus 

 Montagui, Sowerby. A small genus separated from Balanus, on 

 account of the cup-shaped base, but re-united by Sowerby, who 

 shews, in his Grenera of Shells, that this is a merely accidental 

 circumstance, resulting from the situations in which the shells 

 acquire their growth. If, for instance, the Balanus be attached 

 to 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 sub- 

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

 The Acastfe are found imbedded in sponges. Also found in the 

 Pacific ocean and Philippines. Ex. Balanus Montagui, of Great 

 Britain, PI. i. fig. 26. 



ACAVUS. Montfort, 1810. Fam. Limacinea, Blainville ; Coli- 

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

 considered synonymous with De Perrusac's sub-genus Helico- 

 gena. De Montfort has given H. Hgemastoma, as an example, 

 Plate xiii. fig. 267. 



ACCESSARY VALVES, are the smaller or less important testa- 

 ceous 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 Linnaeus and 

 Bruguiere among multivalve shells. 



ACEPHALOPHOEA. BlainviUe. («, without ; /ce^aXf, 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, E-udistes, Lamellibranchiata, and 

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

 ceous MoUusca. 



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

 an agate.) Fam. Limacineas, Bl. Gen. PoltphemijS, Montf. — 

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



