MOLLUSCOIDEA — BRACHIOPODS 1 9 1 



Brachiopod shells are small, averaging from a half inch in 

 length and breadth to an inch and a half. The largest species 

 known — Prodiictus giganteus of the Mississippian — reaches 

 at times a width of almost a foot, while mature forms are known 

 which are no larger than a pinhead. 



There are about 160 species of living brachiopods, which 

 are distributed in ^t^ genera. Over 75 per cent of these species 

 belong to the Articulata. 



Living brachiopods are found in all seas and down to depths 

 of 17,000 feet, hence at all oceanic temperatures ; about 20 per 

 cent live in the shallow seas around Japan, the most prolific 

 area for these shells. Over 70 per cent live in shallow waters, 

 i.e. from between tides to a depth of 600 feet. The great ma- 

 jority (75 per cent) of the Inarticulata live above the 15-fathom 

 line, while but 20 per cent of the Articulata live here. Com- 

 pared with the deep sea species, the shallow water and littoral 

 forms are much more prolific in numbers, with much thicker 

 and usually larger shells. Brachiopods are thus very important 

 in the study of ancient geography. For example, the presence 

 of fossil Inarticulata, especially those of the Lingula group, is 

 good evidence of shallow water deposits. Of those species be- 

 longing to groups existent in the Paleozoic not one is known 

 which at present has a wholly abyssal habitat ; most of the 

 abyssal forms date from stocks having their rise in the mid- 

 dle Mesozoic or later. Hence the conclusion, corroborated by 

 other classes of animals, that abyssal seas began to form at the 

 close of the Paleozoic with the late Paleozoic upheavals, known 

 for North America as the Appalachian Revolution. 



Though all brachiopods are strictly marine in habitat, some 

 forms, such as Lingula, can endure a considerable amount of 

 fresh, water, living in estuaries where there are heavy spring 

 freshets, but they never live in rivers. 



The living inarticulate brachiopods possess a wonderful vital- 

 ity. Lingula may be exposed between tides, covered with mud 

 from freshets, or immersed in fetid water for months without 



