76 ATTACHED. 



ASTROLEPAS. Klein. Coronula Testudinaria, Auct. Chelo- 

 nobia, Leach. Fig. 15. 



ATLANTA. Lesueur. Fam. Pteropoda, Lam. and Bl. — Descr. 

 Spiral, convolute, transparent, fragile, compressed, with a broad, 

 fimbriated, dorsal keel, and a narrow aperture. This shell, 

 which is called *' come d'ammon vivant," is found in the 

 Atlantic. The small Pteropod, figured in Sowerby's Genera as 

 Limacina, belongs to this genus. Atlanta Helicialis, fig. 220. 



ATRACTODON. Charlesworth. (Mag. Nat. Hist. 2nd series, Vol. 

 1. p. 218.) A genus proposed for the admission of a singular 

 fossil shell, found on the beach at Felix-stone, of which the 

 following are the characters ; — fusiform, aperture equalling the 

 spire in length, terminating anteriorly in a slightly recurved 

 canal; columellar lip smooth, curved, thickened posteriorly into 

 a blunt tooth ; spire obtuse. — Obs. This shell would be a Fusus 

 were it not for the tooth on the posterior extremity of the colu- 

 mellar lip. The only species known is regularly striated in a 

 spiral direction, and named A. Elegans. 



ATRYPA, Dalman. A genus of brachiopodous bivalves, dis- 

 tinguished by the valves being nearly equal, and the umbones 

 not separated by an intermediate area. A. reticulata, fig 302. 



ATTACHED. Shells are attached to marine substances by various 

 means ; in some cases by a byssus, or a bunch of tendinous 

 fibres passing through an opening between the valves, which gape 

 at their margins to admit a free passage, as in the genera Bysso- 

 arca and Mytilus. In other cases the byssus is of a more compact 

 substance, and passes through a perforation in the shell itself. 

 This is the case with many of the brachiopodous shells, in some 

 species of which the perforation is in the point of the umbones, 

 a specimen of which is represented in the Introduction. This 

 species of attachment does not keep the animal motionless, 

 although it is confined to a particular spot. Other shells are 

 attached by a portion of their own substance, as in Chama, 

 Spondylus, Serpula, &c. in which instances, the attached valve is 

 motionless, and is termed the under valve. The Pedunculated 

 Cirripedes are attached by a tubular tendinous ^process, called 

 a peduncle. 



