BRACHIOPODA. 



341 



casionally the small one) furnished with an area divided by 

 a deltidium ; both valves articulating by means of teeth 

 and sockets; the former situated on each side of the 

 base of the deltidium of the large valve, and the latter on 

 each side of the boss or cardinal muscular fulcrum of the 

 small valve. — Fossil. 



This genus differs from Productus in having an area 

 and deltidium to one valve ; it is, however, not generally 

 adopted. 



Chonetes. Fischer. — Shell free, testaceous, of per- 

 forated texture, oval, transverse, depressed, inequivalve ; 

 large valve convex, with a smooth, straight, narrow area, 

 interrupted by a projection of the other valve ; and with 

 tubular spines placed on the exterior edge of the area at 

 regular distances ; small valve concave, with a small area ; 

 hinge line straight. — Fossil. 



Aulosteges. Helmersen. — Shell with both valves 

 convex as in Or this, the hinge usually the whole breadth 

 of the shell ; dorsal area high, with a small opening covered 

 by an arched deltidium; ventral area rudimentary, with a 

 small deltidium which surrounds the base of an elevated 

 double tooth ; both valves set close with hollow spines, — 

 Fossil. 



Lepmna. Dalman. (Plectambonites and Ortham- 

 bonites Pander.) — Shell having the hinge area in each 

 valve extending the whole width of the shell, and the 

 margins of the valves produced together beyond that part 

 of the shell which contains the animal, and bent down so 

 as to form a kind of inverted cup; the flatter (lower) 

 valve has no projecting beak, but in the middle of its 

 hinge-line, is a process which nearly fits the aperture in 

 the convex deltidium of the other valve, which slides upon 



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