143 



hinge composed of compressed plicately channelled cardinal 

 teeth, and, on either side, compressed lateral teeth, enclosing a 

 pit containing the ligament. 



Although the genus Mactra, of which we have five well-marked species 

 on our own shores, has been split up into half-a-dozen genera, there are 

 few lamellibranchiate groups of which it is more easy to determine the 

 generic characters. The animal has its mantle open in front as far as the 

 siphons, and the siphons are more or less distinctly serrately fringed ; the 

 shell has with little variation a characteristic triangular form, more or less 

 transversely ovate or oblong, and the hinge is distinguished by a conspicu- 

 ous central triangular tooth in one valve, which interlocks unusually closely 

 with the opposite valve. The Mactra shells are mostly devoid of colour, 

 but some are very prettily tinged and rayed with rose or purple. In sculp- 

 ture they are somewhat wanting, having seldom more than concentric 

 grooves and striae or plait-like ridges. The anterior or front half of the 

 shell is, with few exceptions, the shorter, and this is sometimes concentri- 

 cally grooved and striated, while the posterior side is smooth. Beneath 

 the uinboes on the anterior side a neat heart-shaped lunule is generally 

 formed, and the striae of the valves mostly converge on the shield of this 

 in a more regularly and gracefully developed manner. On the posterior 

 side the ligamentary area is more broadly spread, it is mostly angled on 

 each side. 



Some of the Mactra, more especially those inhabiting the Northern 

 Seas, are ponderous and Mya-\\ke ;* the majority of the species inhabiting 

 tropical seas are thin, some almost transparent, and most elegantly wave- 

 wrinkled and striated. The genus is widely distributed over the globe, 

 from the Arctic Seas, along both sides of the African continent to Tas- 

 mania in one hemisphere, and from North America to Panama and Colum- 

 bia in the other, and it is more than probable that its southern limit is not 

 as yet known. 



Species. 



1. achatina, Chemn. 3. /Egyptiaca, Chemn. 5. alata, Speug. 



2. Adansoni, Phil. 4. sequilatera, Desh. 6. Anatiuoides, Reeve. 



* Speaking of the Mactra solidissima of Massachusetts, Dr. Gould says, "This is character- 

 ized at maturity hy its great magnitude, which entitles it to the name of Giant Clam. No other 

 species of the genus approaches it in size, except M. ovalis. The largest specimen I have seen 

 measures 6£ inches in length, 4 in height, and 2^ in breadth. It is found about sandy beaches on 

 all our coast, and its inhabitant is much esteemed by some as an article of food. At low-water 

 it is dug out of the sand with shovels. At higher tides, when the shell is open, the fishermen 

 wade into the water, thrusting a pointed stick into the sand before them as they walk along. If 

 the stick happens to pass between the valves, they are closed upon it by the animal, and the 

 shell is thus drawn up." — Invert. Massachusetts, p. 52. 



