BRACHIOPOUA. 



667 



Merista (fig. 527, f), of the Silurian and Devonian, is like Athyris in 

 general characters, but there is a longitudinal septum in the ventral valve, 

 which is supported by strongly-arched transverse plates, together form- 

 ing what is known as the " shoe-lifter process." Meristella (fig. 527, G 

 and H, and fig. 529) closely resembles the preceding, but the septum and 

 supporting arched plates are wanting in the ventral valve. The genus is 



Fig. 529. 



-a, Cast of the interior of the ventral valve of Meristella nasnta, from the Devonian 

 (Original); b, Interior of the ventral valve of the same (after Billings). 



Silurian and Devonian, and a well-known and familiar species is the 

 Meristella tumida of the Silurian rocks. 



In the Silurian and Devonian genus Nucleosftira, the shell has a punc- 

 tated structure, with a short hinge-line, and a false area with a minute 

 peduncular foramen beneath the beak of the ventral valve. There is a 

 low median process in the interior of the ventral valve ; and the dorsal 



Fig. 530. — 1. Interior of a specimen of {Incites gryphus, from the Middle Devonian, in which 

 the greater part of the dorsal valve has been removed. The spires are destroyed, but their 

 "crura" remain. 2. Restored interior of the dorsal valve of the same: a, Cardinal process; 

 b, Principal stems of the spires (d) ; c, Band connecting the "crura" of the spires; e, Pouch- 

 shaped expansions of the beaks. (After Davidson.) 



valve has a spatula-shaped cardinal process, which extends upwards 

 below the beak of the ventral valve, and to the base of which the " crura" 

 of the brachial spires are attached. 



The genus Retzia includes a large number of species which range from 

 the Silurian to the Trias. The shell in this genus (fig. 527, a) is oval, 

 usually radiately striate or ribbed, the beak of the ventral valve being 



