532 
GEOGRAPHICAL ZOOLOGY. 
[part IV 
Class.— BRACHIOPODA. 
These are sedentary, bivalve, marine mollusca, having laterally 
symmetrical shells, but with unequal valves. Both in space and 
time they are the most widely distributed molluscs. They are 
found in all seas, and at all depths ; and when any of the families 
or genera have a restricted range, it seems to be due to our im- 
perfect knowledge, rather than to any real geographical limita- 
tions. In time they range back to the Cambrian formation, and 
seem to have had their maximum development in the Silurian 
period. It is not, therefore, necessary for our purpose, to do 
more than give the names of the families with the numbers of 
the genera and species, as before. 
Family 1. — TEREBRATULlDiE, (5 Genera, 67 Species 
living, 340 fossil.) 
Family 2. — SPIRIFERIDAE. (4 Genera, 0 Species living, 
380 fossil.) 
Family 3. — EHYRCHORELLIBJE. (3 Genera, 4 Species 
living, 422 fossil.) 
Family 4. — ORTHIDiE. (4 Genera, 0 Species living, 328 
fossil.) 
Family 5. — PRODUCTIDiE. (3 Genera, 0 Species living, 146 
fossil.) 
Family 6. — CR ANIADiE, (1 Genus, 5 Species living, 37 
fossil.) 
Family 7. — DISCINIDiE. (2 Genera, 10 Species living, 90 
fossil.) 
Family 8. — LINGULIDiE. (2 Genera, 16 Species living, 99 
fossil.) 
