64 



the Mollusca of the Pacific coast of America from that of the Atlantic, must 

 admit that in this family, at least, no such barrier exists. This wide distribu- 

 tion has doubtless been caused, in a great degree, by the circumstances of 

 habitation of several of the species, which seem to select floating timber for 

 their abode. In these habitations they appear to sustain those vicissitudes of 

 temperature which so generally circumscribe the Marine Testacea, except 

 deep-sea species, to restricted zones of latitude ; but it is exceedingly surpris- 

 ing that the larger species, which naturally make their abode in stone or mud, 

 do not appear to be any more restricted in habitat than the others. In illus- 

 tration of this subject, the following species and their range are cited : 



Pholas costata, L. New Bedford, Mass. Mexico. Mediterranean. 



Pholas truncata, Say. Atlantic coast from Nova Scotia to Florida. Peru. 

 Chili. 



Zirphaja c r i s p a t a, L, Europe. United States. West coast (teste Car- 

 penter.) 



Martesia s t r i a t a, L. Europe. "West Indies. Philippine Islands. 



The manner in which the animals of Pholas excavate the holes in rocks, 

 wood and bard clay, in which they reside, has long proved a puzzling ques- 

 tion to naturalists, and various theories have been started in explanation. 

 The hypothesis of the evolution of an acid or solvent to eat away the surface 

 of limestone rocks, was met with the powerful objection that the delicate 

 valves of the animal itself would be equally liable to attack, and when it was 

 found that the Pholas, not restricting its operations to carbonate of lime, exca- 

 vated with equal facility surfaces on which acid has no effect, — gneiss, for in- 

 stance, — the "solvent theory" received its death-blow. The use of the valves 

 with their sharp imbrications in effecting the work of excavation is forbidden 

 by their frequently perfect state, even when contained in the hardest sub- 

 stances ; — (exemplified by a piece of extremely hard gneissic rock from the 

 coast of France, containing a magnificent specimen of Dactylina dactylus, 

 with its imbricated ribs sharp and perfect. Coll. Acad. Nat. Sci.) 



The anterior part of the animal of Pholas has a granulated surface, caused 

 by the presence of numerous siliceous particles ; and this is probably the in- 

 strument which the animal employs in its work. Recent investigations have 

 shown that the^e granules are renewed as fast as they are worn off by attrition 

 with the surrounding surface, thus forming an analogy with the tongue of the 

 Gasteropoda. The young shells of Pholadidae frequently differ much from the 

 adult, and this difference has caused the description of many of these as dis- 

 tinct species ; the synonymy of the family is further confused by the redescrip- 

 tion of species procured from stations far distant from the original localities. 



Dr. Gray includes in the family Pholadidffi, three subfamilies, which are thus 

 characterized : — 



1. Pholadinje. Dorsal muscle attached by one or two dorsal shelly valves. 

 Cavity in which the animal lives not lined with a regular shelly tube enclosing 

 the valves. 



2. Zirphjeina. Dorsal muscles only covered with a horny or coriaceous 

 epidermis. The cavity in which they live not lined with a regular shelly tube 

 enclosing the valves. 



3. Teredinina. Dorsal muscles covered with a coriaceous epidermis. Cavity 

 in which they live lined with a regular shelly tube surrounding the valves. 



The great differences between Pholas and Teredo (strengthened by Dr. 

 Gray's recent discoveries respecting T. giganteus) have induced me to 

 separate them into distinct families, one containing two, the other three sub- 

 families, as follows. 



