BRACHIOPODA. 447 



both genera the beak of the. ventral valve is really perforated, 

 in the young state at any rate. Some authorities, however, 

 retain the name of Spirigera for those forms in which the 

 foramen remains throughout life, and employ that of Athyris 

 for those in which this aperture becomes closed in the 

 adult condition, the latter having the additional distinctive 

 character that the interior of the dorsal valve is partially 

 divided by a longitudinal septum.) The spiral supports for 

 the arms in Athyris are largely developed, and their pointed 

 extremities are directed towards the lateral angles of the 

 shell (fig. 284, c). Merista (fig. 290, F), of the Silurian and 

 Devonian, is like Athyris in general character, but there is a 

 longitudinal septum in the ventral valve, which is supported 

 by strongly-arched transverse plates, together forming what 



Fig. 291. a, Cast of the interior of the ventral valve of Meristella nawta, from the Devonian 

 (original) ; 6, Interior of the ventral valve of the same (after Billings). 



is known as the "shoe-lifter process." Meristella (fig. 290, 

 G and H, and fig. 291) closely resembles the preceding, 

 but the septum and supporting arched plates are wanting 

 in the ventral valve. The genus is Silurian and Devonian, 

 and a well-known and familiar species is the Meristella 

 tumida of the Upper Silurian. 



The genus Retzia (fig. 290, A), with a geological distribu- 

 tion from the Silurian to the Carboniferous, has the spiral 

 brachial supports of the Spiriferidce, but the foramen is 

 large, and the shell resembles that of Terelratula. The shell- 

 structure is punctate, and there is a small hinge -area. 

 Uncites (fig. 290, B), of the Devonian, resembles the preced- 

 ing, but the beak of the ventral valve is strongly curved, the 

 foramen disappears early, there is no hinge-area, and the 

 shell-structure is impunctate. 



