106 CLASS I. TROPIOPODA. ORDER I. BIMUSCULOSA. 



shelf. The ligament is external, attached to a part of the area between 



the umbones. 



Examples. 



PI. LXXXI. Fig. 1,2 and 3. 

 Cucull.ea auriculifera, Lamarck, Anim. sans vert., new edit., vol. vi. 



p. 455. Enc. Me'th., pl. 304. f. 1. a, b, c. 

 Area cucullus, Gmelin. 

 Area cucullata, Chemnitz. 

 Area concamerata , Martini. 

 Area concamera, Bruguiere. 

 Area auriculifera, De Blainville. 



PI. LXXXI. Fig. 4. (fossil.) 



Cucull.ea decussata, Park., Org. Remains, vol. iii. p. 171. Sowerby, 

 Min. Conch., pl. 20G. f. 3 and 4. Deshayes, new edit, of Lamarck, 

 vol. vi. p. 456. 



ARCA, Linnaeus. 



Testa sequivalvis aut subrequivalvis, insequilateralis, trapeziformis, plus 

 minusve ventricosa, epidermide pilosa plerumque induta ; umboni- 

 bus distantibus, incremento arese ligamenti gradatim discedentibus. 

 Cardo linearis, rectus, dentibus minimis, plurimis. Ligamentum 

 externum ad aream inter umbones affixum. Animal interdum bysso 

 instructum. 



The genus Area is very numerous in species, and may be easily recog- 

 nised by the distinct and never-failing character which its shell exhibits 

 in the number and rectilinear position of the teeth. Some of the Arcae 

 are characterized by having a byssus ; but, although a slight aperture or 

 the indication of one is generally left for the passage of it between the 

 edges of the valves, this feature is difficult to be ascertained, and cannot 



