82 PAL/EONTOLOGY 



latitudes, only a few stragglers being found in British 

 strata (Toncasia lonsdalei in the Lower Greensand of 

 Wiltshire only ; Durama mortoni and a few others in the 

 Chalk). In the Mediterranean region they serve as zonal 

 indices, particular species having a narrow range in time, 

 contrary to the general rule among lamellibranchs. At 

 the end of the Cretaceous period the Rudistes died out. 



3. Lucinacea. Ornament concentric, sometimes 

 combined with radial. Trias, to Recent. Forms closely 

 resembling Lucina (but hinge unknown) occur in Silurian 

 and Devonian. The chief genera are the nearly circular, 

 usually thin Lucina (Fig. 24, a), with a long and narrow 

 anterior adductor (numerous sub-genera, Trias.-Rec.), the 

 stout Unicardium(Tria.s.-Cret), Corbis (Jur.-Rec.) in which 

 a fine radial ornament is seen in the depressions between 

 the strong concentric ridges, and Diplodonta (Eoc.-Rec.) 

 in which the teeth 2 and 3^ are bifid. 



4. Chamacea. Fixed forms, inequivalve, closely 

 analogous to the simpler Rudistes, but derived at a later 

 period probably from Lucinacea. 



The only genus is Chama (Fig. 24, d, Cret.-Rec.), which is 

 fixed usually by the left valve (some species by the right). 

 The umbones are strongly prosogyral, the fixed valve 

 larger and the free valve lid-like. The exterior has a 

 scaly appearance. 



5. Mactracea. Derived from Diplodonta (Lucinacea), 

 with which they agree in having tooth 2 bifid, and differ 

 in being sinu-palliate and having the ligament internal 

 (resilium). Ornament concentric, striate. 



Mactra (Fig. 24, c, Eoc.-Rec.) is oval in form. Ensis (the 

 razor-shell, Rec.) has taken to burrowing, and so has 

 acquired the oblong shape common among Desmodonts 

 (see later). It is, in fact, a homoeomorph of Solen, from 

 which -it is distinguished by its heterodont teeth. 



