240 



NATURAL HISTORY. 



the umbones are not at the extreme termination of the shell, as in Hytilm. Seventy species are found 

 iu tropical seas. 



Lithodomus, the Date-shell, bores into corals and even hard limestone rocks. The animal, which 

 is like the common Mussel, is eaten in the Mediterranean. Perforations made by Lithodomi in lime- 

 stone cliffs and in the columns of the Temple of Serapis, at Puteoli, have afforded conclusive evidence 

 of changes in the level of the sea-coasts in modern times. (Lyell.) 



THREE ERECT COLUMNS OF THE TEMPLE OF SERAPIS AT TUTEOLI 130RED BY LITHODOMI. 



Genus Dreissena. The animal bears a triangular fan-shaped shell ; the mantle is closed through- 

 out, except for the passage of the foot, and of two tubular orifices for the purposes of excretion and 

 respiration. The right valve has a slight byssal sinus. " Dreissena polymorpka is a native of the Aralo- 

 Caspian rivers. In 1824 it was observed by Mr. J. Sowerby in the Surrey Docks, to which it appears 

 to have been brought with foreign timber in the holds of vessels. It has since spread into the 

 canals, docks, and rivers of many parts of England, France, and Belgium, and has been noticed 

 in the iron water pipes of London." (S. P. Woodward.) 



FAMILY VI. ARCADE. 



The shell is covered with a strong epidermis hinge line, often elongated and toothed ; the valves 

 are tumid and equal. The foot of the animal is large, curved, and deeply grooved. 



Genus Area. The "Arks" have thick, inflated, ribbed, and striated shells; the umbones anterior 

 are divided by a lozenge-shaped hinge area ; the foot is long and pointed, the mantle bears ocelli on its 

 border. One division of the Arcs (Byssoarca} has a wide byssal aperture, filled with a horny cone ; 

 these conceal themselves under stones at low water, in crevices of rocks, and the empty burrows of 

 boring molluscs. They inhabit all the warm seas of the globe, from low water to more than 200 

 fathoms. One species lives in the Ganges, 1,000 miles from the sea. (Benson.) 



