BRACHIOPODA. 



345 



no hinge ; the substance of the shell is more horny than 

 calcareous, and is even flexible when moist. 



Discina. Lam. (Orbicula Owen ; not Cuvier.) — Shell 

 horny, orbicular, rather irregular, inequi valve, rather com- 

 pressed, the upper valve like a Patella, the 

 lower flat, and attached ; in the centre of 

 the latter is a small oval depression, with 

 an oblique fissure in it for the passage 

 of a tendon ; four muscular impressions in 

 each valve ; no hinge or ligament. Animal 

 having two short ciliated arms, and a muscle 

 or ligament which passes through a fissure 

 in the shell and adheres to other sub- Discina. 

 stances. — Few species ; also fossil. 



These shells are found in the Northern Seas, also in 

 those of Peru and Chili ; the individuals are often found 

 grouped on each other. 



Siphonotreta. De Verneuil. — Shell inequivalve, 

 equilateral, texture rather horny ; valves with free teeth, 

 not articulated ; large valve convex, the boss submarginal, 

 and pierced with a rather round hole, which is oblique ; 

 the small part of the valve, comprised between the boss 

 and the cardinal edge, has neither perforation nor delti- 

 dium, and is covered with lines of growth which surround 

 circularly the boss, as in the genus Orbicella : the opening 

 at the apex has an open tube which is prolonged into the 

 centre of the shell ; the smaller valve is oval, and is a little 

 less convex than the opposite valve. — Fossil. 



Trematis. Sharpe. (Orbicella U Orb. Schizotreta 

 Kutorga.) — Shell suborbicular, inequivalve, attached by 

 a ligament passing through a longitudinal fissure in the 

 posterior part of the ventral valve ; valves united by 

 a hinge which is supposed to resemble that of Terebra- 



