532 GEOGKAPHICAL ZOOLOGY. [pabt 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,— TEREBRATULIDiE. (5 Genera, 67 Species 

 living, 340 fossil,) 



Family 2.— SPIRIFERID^. (4 Genera, Species living, 

 380 fossil.) 



Family 3,— RHYNCHONELLIDiE. (3 Genera, 4 Species 

 living, 422 fossil.) 



Family 4.— ORTHID^. (4 Genera, Species living, 328 

 fossil.) 



Family 5.— PRODUCTID^. (3 Genera, Species living, 146 

 fossil.) 



Family 6,— CRANIAD^, (1 Genus, 5 Species living, 37 

 fossil.) 



Family 7.— DISCINID^, (2 Genera, 10 Species living, 90 

 fossil.) 



Family 8.— LINGULID^. (2 Genera, 16 Species living, 99 

 fossil.) 



