54 HANDBOOK OF SHELLS. 



2. The Mactrida are somewhat triangular in shape, and 

 may be at once recognised by the 

 pit for the hinge-ligament, which 

 also assumes that form, as seen in 

 the accompanying figure of Lntraria 

 pi. 21. 



21). 



3. The Tellinida comprise some of the most delicately tinted, 

 both externally and internally, of all shells. In some, coloured 

 bands radiate from the umbones, and well bear out the fanciful 

 name of "Sunset shells" bestowed upon them. Their valves 

 are generally much compressed. 



4. The Solenidcc, or " Razor- shells," rank next, 

 and are readily recognised by the extreme length of 

 the valves in proportion to their width, and also by 

 their gaping at both ends. 



J. The Myaddce, or " Gapers," have the siphonal 

 s wide apart (in the genus Mya botli ends 

 gape), and are further characterized by the triangular 

 process for the cartilage, which projects into the 

 interior of the shell. One valve (the left) is gene- 

 rally smaller than the other. 



6. The Anatinidcc have thin, often inequivalve 

 pearly shells. The genus Pandora is the form most 

 frequently met with in collections. 



7. The Gastrochcenidce embraces two genera (Gas- 

 trachoma and Saxicava} of boring mollusca, which 

 perforate shells and rocks, and also the remarkable 

 tabe-like "Watering-pot-shell" (Aspergilhtm] which 

 is hardly recognisable as a bivalve at all. 



8. The Pholadidce concludes the list of bivalves, 

 and comprises the common rock-boring Pholas 

 (Fig. 22) of our coasts and the wood-boring ship- 

 ivorm "Teredo" (Fig. 23). 



Although the Brachiopoda, or " Lamp-shells," are 

 not true mollusca, they are not very far removed from 

 them, and are so often to be found in cabinets that 

 it will not do to pass them over, especially since in 

 past times they were very abundant, an enormous 

 number occurring in the fossil state. Only eight 

 genera are now living. Shells belonging to this 

 class are readily recognised by the fact of one valve 

 being larger than the other, and possessing a distinct 



