MOLLUSCA. 59 



are obsolete, the ligament is external, and the shell gaping. The shell presents 

 many distorted varieties, and some species excavate cavities in rocks. Some 

 authors unite this family to the Lithophaga, among the Elatobranchia. 



Fam. 5. Pandoridce. A small family formed of the single genus Pandora, 

 which has only a single branchia on each side. The shells are small, 

 pearly within, the valves unequal, the ligament interior, the siphons short, 

 the hinge with two diverging teeth in the right or flat valve, and two 

 corresponding impressions in the left, which is convex. P. trilineata^ Say 

 {American Concliology^ 2:>l. 2), inhabits the American coast from Maine to 

 Florida. This genus is by some placed with the Solenida?, and by others 

 with the Myida3. 



Fam. G. Solenidm. This family is composed of the genera Solen 

 {pi. Y6, figs. -15-6), Solecurtus, Glyciraeris, I^nopsEa, and Pholadomya. 

 The ligament is external, the shell gaping at both ends, and the foot is long 

 and fleshy, and is passed out of the anterior extremity. The genus Solen 

 lives in holes several feet deep, in which it passes up and down by means 

 of its foot, and with such rapidity that it is difficult to take. Fishemnen 

 take them for bait by suddenly thrusting a barbed spear into their retreat, 

 but if they are not struck at once they descend beyond reach. The genus 

 Glycimeris is remarkable for having the body so large in proportion to the 

 shell, that the latter is unable to inclose it. G. siliqua, the only species of the 

 genus, as now restricted, inhabits the banks of ]^ewfoundland^ and the shell 

 is usually got from the stomachs of cod-fish. Panoptea is a genus including 

 species mostly of a large size, few in number, and occurring recent and fossil. 

 P. arctica inhabits the banks of Newfoundland, and about eiglit fossil species 

 have been named from the tertiary formations of the United States. 



Fam. 7. PJioladidcB. This family contains various genera, wliich live in 

 perforations wliich they make in rocks, wood, or clay, each being a prisoner 

 for life in a cell of its own construction. In Pholas {pi. 76, Jigs. 49, 50) 

 the shell is widely gaping, and provided with accessory pieces at the hinge 

 {Jig. 4:9), and the foot is shaped like a sucker. Pholas daetylus {fig. 50) 

 is used as food. A few fossil species are found in the tertiary formations 

 of Europe and America. The genus Teredo is placed by some authors in 

 the next family, Tiibicola.^ on account of its form ; but Deshayes thinks the 

 characters of the animal require that it should be placed here. Teredo 

 navalis {p>l. 75, fig. 67), which is so destructive to timber and shipping 

 when not protected, appears as a long, sinuous, calcareous tube, lining 

 perforations in wood, and if one of these be followed to its lower end, the 

 valves of the shell will be found. The shell is thick, and so short as to be 

 annular, having a single adductor mus51e, and the posterior extremity of the 

 tube is open. According to Laurent, when the young Teredo escapes from 

 the egg, it swims about by means of vibrilla?. At first the body is inclosed 

 in the shell proper, and the little animal can move about with the aid of a 

 long linguiform foot. After spending about twenty-four hours in swimming 

 and moving about upon submarine objects, it attaches itself to wood, 

 making a depression in which it locates itself, and excretes a mucous mass 

 which covers it, but allowing a passage for the siphons. The }'oung animal 



'♦263 



