BRACHIOPODA. 123 



extends from the umbone almost to the front of the valve, 

 and increases in height towards the latter. Dorsal valve 

 less convex, with a small septum (), and a long slightly 

 curved cardinal process (f), divided at its extremity to 

 embrace the ventral septum. The brachial skeleton 

 consists of two branches arising from the hinge-plate (p), 

 which pass to the middle of the shell (c) and are then 

 sharply bent back and form a ribbon (I) parallel to the 

 margin of the valve. Silurian and Devonian. 



Distribution of the Brachiopoda. 



The Brachiopods are all marine and are found in all 

 parts of the world ; at the present time they are much 

 less numerous than in former periods of the earth's history. 

 About half of the existing species are found at depths 

 of less than 100 fathoms, several of these do not extend 

 beyond this limit. Below 150 fathoms they soon become 

 comparatively rare, but a few species occur down to 2900 

 fathoms. Some genera, as for instance Lingula, do not 

 occur below a depth of a few fathoms. A few species, as 

 Terebratulina caput-serpentis, have a very wide geograph- 

 ical distribution, extending from polar to tropical regions, 

 and these have also a great range in depth, the form 

 mentioned being found from the shore down to 1180 

 fathoms. 



Generally the species confined to shallow water have 

 a much more limited geographical range than those found 

 in deeper water ; and the polar or boreal species have a 

 wider range than the tropical, since they can find the 

 same temperature in lower latitudes at great depths. 



The Brachiopods are very abundant as fossils, the 

 earliest forms occur in the Lower Cambrian beds. In the 



