460 MARINE INVERTEBRATES 
united siphons, protected laterally at their larger end by two white, 
widely gaping valves. The very small pedal opening in the line of 
mantle juncture indicates a very small foot. (Plate LXXXIV.) 
FAMILY PHOLADIDE 
There is a striking resemblance between the shells of this rock- 
boring family and those of the petricolas—a resemblance which 
consists principally in the texture of the shell and the tendency 
to different types of sculpturing upon the posterior and anterior 
surfaces of the valves. 
Genus Pholas 
In Pholas the anterior part of the hinge-margin is reflected 
back over the umbones, and a long rib-like tooth springs from 
under the umbonal region and curves out almost to the center of 
the valves. The shell gapes “fore and aft,” and is thin, white, 
very hard, and brittle. The animal has a short, truncated foot 
and a small orifice in the mantle through which the foot may be 
projected. The siphons are long and are united almost to their 
ends. Dorsally, an exposed portion of the animal is protected by 
accessory calcareous plates. The habits of Pholas are very inter- 
esting. It is found in holes gouged out of solid rock or out of 
pieces of wood; sometimes, like Petricola, it may be found exea- 
vating in hard clay. As the anterior end of a Pholas shell is the 
larger in diameter, and is the end which is most deeply buried in 
the rock (the opening of the burrow being comparatively small), 
there was some mystery as to the way in which the creature man- 
ages to get inside its rocky dwelling. Observations in aquaria 
have shown that the young Pholas begins his process of home- 
building very early in life. The wearing away of the stone is 
effected by constantly turning the shell around, scraping with 
the anterior edges of the valves. A lodging-place having been 
secured, Pholas is obliged to work constantly to enlarge his bur- 
row for the accommodation of his growing shell. The long siphons 
may project from the hole and draw in food and breath, but the 
animal is a prisoner for life. When Pholas has withdrawn his 
long siphons he is reasonably secure from his enemies, but 
