294 MOLLUSCA FROM THE CRAG. 
In some specimens, the terminal tube projects a little distance beyond the surface 
of the coral, and of course, when alive, has its siphons always free. Occasionally the 
coral has either grown faster than the mollusc, or, what is more probable, had con- 
tinued to increase after the death of the imprisoned animal, the terminal opening having 
been closed up by the growth of the coral. 
PHOLAS,* Linneus. 
Dactrytus. Pliny. | Barnia. Leach, MS. 1819. 
Hypoc#a HypocroprrMa. Poli, 1791. Dactytina. Gray, 1847. 
Zinrma. Leach, MS. 1817. Gray, 1851. ANCHOMASA. Leach, MS. 1819. 
THovana. Id. 1818. 
Generic Character. Shell ovate or transversely elongate, equivalved, inequilateral, 
externally rough or imbricated, more or less ornamented with radiating ridges, gene- 
rally gaping at both extremities, occasionally furnished with accessory valves, and a 
reflected callosity over or before the beaks; a large, curved, testaceous appendage 
projects immediately beneath the umbo; no teeth or hinge ligament; and the impres- 
sion of the mantle is deeply sinuated. 
Animal thick and club-shaped; lobes of the mantle open in front, and reflected 
dorsally, by which the accessory valves are formed; siphonal tube long and extensile, 
divided at the extremity, and bordered with cirrhi; foot thick and truncated. 
This is, generally speaking, a marine genus, and most of the species are inhabitants 
of shallow water, ranging to about 25 fathoms. Ph. rivicola, Reeves and Adams, 
‘Voyage of the Samarang,’ pl. 25, fig. 5, is said to have been found burrowing in 
floating logs at Gunung Taboor, on the Pantai River, twelve miles from the sea, and 
where the water was quite fresh. This species is divided by a depressed line or sulcus, 
like PA. crispata, and has, at the pedal opening, a calcareous covering, but without an 
accessory cup at the base of the siphons. 
It is extremely difficult to define the generic limits of this group of shells. Some 
are furnished with an umbonal shield, consisting of several pieces; some have only 
a single dorsal valve, while in others this appendage is rudimentary or absent. These 
differences are considered to be only of specific value with many naturalists, while 
others make them generic distinctions. 
The name given implies that the habits of these animals are those of excavators, 
and they are found to burrow in various materials, sandstone as well as calcareous 
rocks, wood, &c. The species do not always confine themselves to one kind for their 
habitations, and in all probability the mechanical mode is the one most generally 
* Etym. gwXew, to burrow, or hide in a hole. 
