CONCHIFERA. 265 



The common edible mussel frequents mud-banks which are uncovered at 

 low-water ; the fry abound in water a few fathoms deep ; they are full-grown 

 in a single year. From some unknown cause they are, at times, extremely 

 deleterious. The consumption of mussels in Edinburgh and Leith is esti- 

 mated at 400 bushels (=400,000 mussels) annually; enormous quantities 

 are also used for bait, especially in the deep sea fishery, for which purpose 30 

 or 40 millions are collected yearly in the Frith of Forth alone. {Br. Knapp.) 

 Mussels produce small and inferior pearls. At Port Stanley, Falldand Ids. 

 Mr. MacgUiivi-ay noticed beds of mussels which were chiefly dead, beins^; 

 frozen at low-water. 3L bilocularis (Septifer, Uecluz) has au umbonal shelf 

 for the support of the anterior adductor, like Dreissenai it is found at 

 Mauritius and Australia. M. exustus (Brachydontes, Sw.) has the hinge - 

 margin denticulated continuously. 



Bistr, 50 sp. World-wide. Ochotsk, Behring's Sea, Russian Ice-meer; 

 Black Sea, C. Horn, Cape, New Zealand. 



lossik 80 sp. Permian — , U. S. Eui'ope, S. India. 



? Myamna, Koninck, 1842. 



Types, M. Goldfussiana, Kon. Carb. M. acuminata. Shy. Permian. 



Shell equivalve, mytili-form; beaks nearly terminal, septiferous inter- 

 nally; hinge-margin thickened, flat, with several longitudinal cartilage- 

 grooves ; muscular impressions 2 ; pallial line simple. 



Fossil 6 sp. Carb. — Permian. Europe. The ligamental area resem- 

 bles that of the recent Area obliquata, Chemn. India. , 



MoDTOLA, Lam. Horse-mussel. 



Etym. Modiolus, a smaU measure, or drinking-vesseL 



Hx. M. tulipa, PI. XVII. fig. 5. M. modiolus, p. 250, fig. 177. 



Shell oblong, inflated in front : umbones anterior, obtuse : hinge tooth- 

 less ; pedal impressions 3 in each valve, the central elongated ; epidermis 

 often produced into long beard-like fringes. 



Animal with the mantle-margin simple, protruding in the branchial 

 region ; byssus ample, fine ; palpi triangular, pointed. 



The Modiolm are distinguished from the Mussels by their habit of bur- 

 rowing, or spinning a nest. Low- water — 100 fms. 



Distr. 50 sp. chiefly tropical; M_. modiolus, Arctic seas — Brit. 



lossil, 130 sp. Silurian ? Lias — . U. S. Europe, Thibet, S. India. 



Suh-genera. Lithodomus, Cuv. M. lithophaga, PI. XVII. fig. 7. Shell 

 cylindrical, inflated in front, wedge-shaped behind; epidermis thick and 

 dark; interior nacreous.* Distr. 12 sp. W. Indies — New Zealand., 'Fossil, 



* The outer shell-layer has a tubular structure; th« tubes are excessively minute . 

 seldom branching, oblique qnd parallel. {Carpenter.) 



