LAMELLIBRANCHIATA. 



73 



are from the West Indies, Southern Europe, Algeria, and the 

 East. The form which approaches nearest to Ghama is the 

 little genus Gaprotina (fig. 19, 7), whose upper valve has a 

 marginal umbo, but is in other respects like a miniature 

 Racliolite. Cajprina (d'Orb.) has the free valve perforated by 

 canals which open in the inner margin, and in Gaprinella the 

 outer lamina of both valves possesses this structure, One 



Fig. 19. 



Secondary Bivalves. 



1. Diceras ari tinum, Lam. ; Corallian, France. 



2. Requienia ammonia ; Neocomian, S. France. 



3. Monopleura trilobata, d'Orb. ; Neocomian, Orgon. 



4. Hippurites Toucasiana, d'Orb. ; L. Chalk, France. 



5. Radiolites angeiodes, Lam. ; L. Chalk, Gosau. 



6. Caprinella Boissyi, d'Orb. ; L. Chalk, Valley of Alcantara. 



7. Caprotina semistriata, d'Orb.; U. Greensand, Le Mans. 



valve is sometimes spiral (fig. 19, 6), and partitioned off inter- 

 nally by numerous septa, like the water-Spondylus, but so 

 regularly as to resemble the chambered shell of a Nautilus. 

 In the Radiolite (fig. 19, 5), both valves are conical, and the 

 umbo of the free valve (marginal in the very young shell) 

 becomes central in the adult. The structure of the hinge is 

 modified by the absence of any spiralitj- in the valves, but is 

 essentially the same as in Gairrotina and Diceras ; the promi- 

 nent teeth of the upper valve support curved plates for 



