504 FOSSIL BRACHIOPODA chap. 



Family. Obolidae 



Shell varies in shape. Ventral valve provided with pedicular groove 

 or foramen. Cardinal border thickened. No brachial supports. Shell 

 composed of alternating chitinous and calcareous layers. 



(For muscles see p. 496.) 



Range. — Lower Cambrian to Devonian. 



Principal Genera. — Obolus, Obolella, Kutorgina, Linnarssonia, Siphono- 

 ireta, Acrotreta, Neoholus. 



Family. Discinidae 



Shell rounded, valves more or less conical, fixed by pedicle passing 

 through slit or tubular foramen in ventral valve. No calcified brachial 

 supports. Shell structure chitino-calcareous. 



Range. — Ordovician to Recent. 



Principal Genera. — Discina, OrUculoidea, Trematis. 



Family. Craniidae 



Shell calcareous, subcircular ; fixed by surface of ventral valve ; dorsal 

 valve the larger, depressed-conical. Shell structure punctate. 



Four principal muscular scars in each valve, with central triangular pro 

 tuberance in ventral valve (see p. 476). 



Range. — Ordovician to Recent. 



Principal Genus. — Crania. 



Family. Trimerellidae 



Shell thick, calcareous, inequivalve; beak of ventral valve usually 

 prominent; rudimentary teeth maybe present; hinge area well developed, 

 with pseudo-deltidium. In interior of valves muscular platform, "crescent," 

 and sometimes sub-umbonal chambers (see p. 494, Fig. 323). 



Range. — Ordovician and Silurian ; maximum in Wenlock. 



Principal Genera. — Trimerella, Monomerella, Dinoholus, Rhinobolus. 



II. Testicardines 



Family. Productidae 



Shell entirely free, or fixed by ventral valve or spines. Concavo-convex, 

 more or less covered with tubular spines. Hinge line straight. Hinge- 

 teeth absent or rudimentary. 



Cardinal process prominent. 



Reniform impressions in dorsal valve. 



(For muscular impressions see p. 501, Fig. 333.) 



Range. — Silurian to Permian. Genus Productus very characteristic of 

 the Carboniferous. 



Principal Genera. — Productus, Chonetes, Strophalosia, Proboscidella, 

 A ulosteges. 



