04- 



PALEONTOLOGY 



l"), 6), and partitioned off internally by numerous septa, like 

 the water-Spondylus, but so regularly as to resemble the 

 chambered shell of a Nautilus. In the Radiolitc (fig. 1 5, 5), 



Fig. 15. 

 Secondary Bivalves. 



1. Diceras arietinum, Lam. ; Corallian, Fiance. 



2. Ro-quienia 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. Greenland, Le Mans. 



both valves are conical, and the umbo of the free valve (mar- 

 ginal in the very young shell) becomes central in the adult. 

 The structure of the hinge is modified by the absence of any 

 spirality in the valves, but is essentially the same as in Ccqiro- 

 tina and Diceras; the prominent teeth of the upper valve 

 support curved plates for the attachment of the adductor 

 muscles, which become continually more undercut in the 

 course of their growth. In Hifflourites the anterior muscular 

 plate projects horizontally, the posterior vertically, like a third 

 tooth, for which it has been mistaken. In this genus there 

 are two longitudinal inflexions of the outer shell-wall beside 

 the ligamental furrow, one corresponding to the posterior 



