HANDBOOK OF SHELLS. 



7 The Trigoniadce, whose single 

 living genus, the handsome Trigonia 

 (Fig. 1 8), is confined to the Australian 

 coast-line, whereas in times now long 

 past they had a world-wide distribu- 

 tion. 



8. The Unionidce, comprising the 

 fresh-water mussels. 



DIVISION b. SIPHONIDA Integra- 

 pallialia. 



1. The Chamidtz, represented by 



the reef-dwelling Chama. F\s.i&.Trigoniamargaritacea. 



2. The Tridacnidce, whose sole genus Tridacna contains the 

 largest specimen of the whole class of bivalves, the shells some- 

 times measuring t\vo feet and more across. 



3. The Cardiadfe, or cockles. 



4. The Ludnida, in which the 

 valves are nearly circular, and as a 

 rule not very attractive in appear- 

 ance, though the "Basket-shell" 

 (Corbis) has an elegantly sculptured 

 exterior. 



5. The Cycladidce, whose typical 

 genus Cyclas, with its round form 

 and thin horny shell, is to be found 

 in most of our ponds and streams. 



6. The Astartid<z, a family of 

 shells having very strongly de- 

 veloped teeth, and the surface of 



whose valves is often concentrically Fig> ig> H inge of Cardlta 

 ribbed. sinnata. 



7. The Cyprinida, which have very solid oval or elongated 

 shells and conspicuous teeth (Fig. 19). The "Heart-cockle" 

 (Tsocardia) belongs to this family. 



DIVISION c. SIPHONIDA Sinn-palHalia. 

 I. The Venerida. The hard, solid shells of this family are for 

 elegance of form 

 and beauty of colour 

 amongst the most 

 attractive a collec- 

 tor can posses?. 

 Their shells are 

 more or less oval 

 and have three teeth 

 in each valve (Fig. 20) 



Fig. 20. Hinge of Cytherea crycina. 



