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PDF - Wallace Online

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532 GEOGEAPHICAL ZOOLOGY. [part iv.<br />

Class.—BRACHIOPODA.<br />

These are sedentary, bivalve, marine moUusca, having laterally<br />

symmetrical shells, but with unequal valves. Both in space and<br />

time they are the most widely distributed molluscs. They are<br />

found in all seas, and at all depths ;<br />

and when any of the families<br />

or genera have a restricted range, it seems to be due to our im-<br />

perfect knowledge, rather than to any real geogi'aphical limita-<br />

tions. In time they range back to the Cambrian formation, and<br />

seem to have had their maximum development in the Silurian<br />

period. It is not, therefore, necessary for our purpose, to do<br />

more than give the names of the families with the numbers of<br />

the genera and species, as before.<br />

Family 1.—TEREBEATULID^. (5 Genera, 67 Species<br />

living, 340 fossil.)<br />

Family 2.—SPIRIFERID^. (4 Genera, Species living,<br />

380 fossil.)<br />

Family 3.—RHYNCHONELLID^. (3 Genera, 4 Species<br />

living, 422 fossil.)<br />

Family 4.—ORTHID^. (4 Genera, Species living, 328<br />

fossil.)<br />

Family 5.—PRODUCTID^. (3 Genera, Species living, 146<br />

fossil.)<br />

Family 6.—CRANIAD^. (1 Genus, 5 Species living, 37<br />

fossil.)<br />

Family 7.—DISCINID^. (2 Genera, 10 Species living, 90<br />

fossil.)<br />

Family 8.—LINGULID^. (2 Genera, 1 6 Species living, 99<br />

fossil.)

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