Fig. 137.—1. Thracia phaseolina. 2. Thracia pubescens, showing Pallial Line
Some of the common species of this family are popularly known as Lantern shells, and perhaps the most familiar of these is Thracia phaseolina, the specific name of which is given on account of a fancied resemblance of the shell to a bean. The shell is very fragile, and although large numbers may often be seen stranded on sandy beaches, but few of them are perfect specimens.
The family Myacidæ may be recognised by the thick, strong, opaque shells, usually gaping at the posterior end; the wrinkled epidermis which covers the whole or part of the shell; and the united siphons, which are more or less retractile. The mantle cavity is also closed with the exception of a small hole left for the protrusion of the small foot. The pallial line of the shell is sinuated.
Fig. 138.—1. Mya truncata. 2. Interior of Shell. 3. Mya arenaria. 4. Corbula nucleus
In the above illustration we represent the Common Gaper (Mya arenaria), which burrows to a considerable depth in the sand or mud, especially in the estuaries of rivers, from between the tide-marks to a depth of twenty fathoms or more. It may be readily distinguished, in common with the other species of the same genus, by the characteristic wrinkled, membranous tube that encloses its fringed siphons, the membrane being a continuation of the epidermis that extends over the shell. Another characteristic feature of the genus is the large, flat process inside the left valve for the attachment of the internal cartilage. An allied species, Mya truncata, is often found abundantly in company with the above, and may be known by the abruptly squared posterior end.
Other species of the Myacidæ inhabit our shores, including the little Basket shell (Corbula nucleus), the left valve of which is much smaller than the right, which overlaps it. The latter, also, is covered with epidermis, while the former, which is flat, is quite naked.