734 



LAMELLIBRANCHIATA. 



these are directed parallels the surface of the shell instead of perpen- 

 dicular to the surface. Moreover, the intersection of the prisms by hori- 

 zontal or oblique lamellae gives rise to an altogether unique vesicular 

 structure. Further, the prisms in the outer shell-layer in the ordinary 

 Bivalves are solid, whereas in the Hippuritidce they are hollow, and were 

 probably occupied in life by some organic infilling. 



The shell of Hipfiurites (fig. 612) is inversely conical or cylin- 

 drical, and sometimes attains a length of two or three feet. The 

 shell is attached by the larger conical valve, and is closed by a small 

 depressed free valve, with a central umbo. In Radiolites the shell 

 is inversely conical, bi-conical, or cylindrical, with dissimilar valves. 

 The upper valve is sometimes flat, sometimes conical, and has a 



Fig. 614 — Minute structure of the shell of Sph&rulites (Radiolites) Mortoni, from the Upper 

 Greensand, Cambridge, a, Transverse section, enlarged ten times, showing the large hollow 

 prisms of the outer layer (?') transversely divided, the thin inner layer (a) being converted into 

 crystalline calcite ; B, Vertical section, similarly enlarged, showing the prisms of the outer layer 

 with their intersecting cross-partitions. (Original.) 



central or lateral umbo. The external shell-layer in the lower valve 

 is exceedingly thick, and is made up of hollow calcareous prisms, 

 which run parallel with the surface and are intersected by close-set 

 oblique transverse plates (fig. 614); while the inner shell-layer is 

 comparatively thin. In the typical forms of Radiolites the lower 

 valve commonly exhibits on one side two bands running from the 

 beak to the upper margin, which are smooth or striated differently 

 from the rest of the valve. In the closely allied genus Sfiharulites 

 these bands are wanting. 



The Rudistm are not only entirely extinct, but are exclusively 

 restricted to rocks of Cretaceous age, being especially characteristic 

 of the middle and upper divisions of this formation, particular types 

 commonly being confined to special stratigraphical horizons. The 

 Cretaceous deposits of Britain, Europe, Algeria, Asia Minor, Persia, 



