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. (Dr. Knapp.) 

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

 Mr. Macgillivray noticed beds of mussels which were chiefly dead, being 

 frozen at low- water. M. bilocularis (Septifer, Recluz) has an umbonal shelf 

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

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

 margin denticulated continuously, 



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

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



Fossil, 80 sp. Permian . U. S. Europe, S. India. 



? MYALINA, Koninck, 1842. 



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



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

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

 grooves ; muscular impressions 2 ; pallia! line simple. 



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

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



MODIOLA, Lam. Horse-mussel. 



Etym. Modi&lus^ a small measure, or drinking- vessel. 



Ex. 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 Modiola 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. 



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



Sub-genera. Lithodomus, Cuv. M. lithophaga, PL 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; the tubes are excessivaly minute ., 

 seldom branching, oblique and parallel, (Carpenter.) 



