[Illustration: 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. [Illustration: 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. [Illustration: FIG. 139.--_Solen siliqua_ The valves have been separated and the mantle divided to expose the large foot] We now come to the interesting family of Razor shells (_Solenidæ_), specimens of which are washed up on almost every sandy beach, while the living molluscs may be dug out of their burrows at low-water mark. The shells are elongated, gaping at both ends with an external ligament; and the hinge has usually two teeth in one valve and three in the other. The foot of the animal is cylindrical, large and powerful; and the siphons are short and united in the long species, but longer and only partially united in the shorter ones. The gills are long and narrow, and are prolonged into the inhalent siphon.