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 PHOLADID.ZE 



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 exca- 

 vating in hard clay. As the anterior end of a Pliolas 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 When Pholas has withdrawn his 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 



