LECTURES ON MOLLUSCA. 261 



force with which they close the valves makes it. dangerous to put the 

 hand into the open shell. The Clam is considered good eating, and 

 sometimes weighs twenty pounds. The beautiful Hippopus maculatus 

 has no gape for the byssus: it is imported in vast numbers into Liver- 

 pool for parlor ornaments, where duly acidulated specimens can be 

 procured at twelve cents each. These aberrant families make a digres- 

 sion from the main line of the Venus and Cockle group. We return 

 now to the more normal forms. 



Family Lucinid^. 



The shells of this family are either heart-shaped or flattened like 

 Dosinia; but may generally be recognized by the great lengthening 

 of the anterior muscular scar. The mantle is open in front, joined 

 behind to form breathing passages. There is. only one gill on each 

 side, and the mouth and lips are very small. The foot is cylindrical 

 and hollow, often twice as long as the animal. When at rest, it is 

 doubled on itself, and hidden between the gills. Fossil forms are 

 found even in the palajozoic rocks. Lucina proper has lateral and 

 hinge teeth like the Cockles. Some specimens are obliquely sculptured 

 like Strigitla, from which they are known by the mantle-line being 

 without bend. Codakia has a hinge somewhat resembling Dosinia. 

 Loripes has the ligament concealed and no lateral teeth. The animal 

 has a long, fringed excurrent pipe. This is also found in Cvyptodon, 

 where the shell is thin and toothless. 



Fimbria has a stout shell like a transverse Cockle, very beautifully 

 cancellated. There are very few living species, but it abounds fossil 

 from the Lias age. Semicorbis and Sjjhcera have no side teeth. Uni- 

 cardium is almost toothless. The oolitic Tancredia is shaped like 

 Ipliigenia. 



Family Diplodontidje. 



The shells in this family may generally be known by a bifid tooth 

 at the hinge. The animals have two gills on each side, and a tubular 

 foot. Diplodonta has a globular shell, and nestles in crevices. Felania 

 a smooth, flat shell, living in sand. Ungulina has a very irregular 

 ligament, and is said to bore. In Scacchia, the cartilage is internal, 

 and the foot tongue-shaped. It forms a transition to the Kelliads. 

 The shell of Gyrenoida resembles Felania, but the animal is figured 

 with two united, rather long pipes, which however produce no bend 

 in the mantel-line. 



Family KELLiADiE. 



The Kelliads all have thin, small shells, generally with an internal 

 cartilage. The animal has a strap-shaped foot, with which it crawls 

 about, or moors itself by a byssus at pleasure. They generally nestle 

 in holes and crypts, and have been mistaken for borers. Some species 

 have a very wide distribution. They are found fossil in the tertiaries. 

 In Kellia, the ligament interrupts the hinge margin, and the mantle 

 is produced in front into a breathing tube. In Lasea, the ligament 

 lies on the thickened hinge-margiu. In Turtonia, it is external; and 

 in Cyamium partly so. 



