372 



MANUAL OF THE MOLLUSCA. 



Bliell impunctate,* transversely oval or elongated, trl-lobed, 

 beaked, bi-convex, with a dorsal ridge and ventral furrow; 

 hinge-line wide and straight ; area moderate, striated across ; 

 foramen angular, open in the young, afterwards progressively 



Dorsal valve. 



Fig. 168. 



Ventral valve. 



closed ; ventral valve with prominent hinge -teeth, and a central 

 muscular scar, consisting of the single adductor flanked by two 

 cardinal impressions ; dorsal valve with a small cardinal process, 

 a divided hinge-plate, and two conical spires dii^ected outwards 

 and nearly filling the cavity of the shell ; crura united by an 

 oral loop. The shell and spires are sometimes silicified in lime- 

 stone, and may be developed by means of acid. In 8. mosquensis 

 the dental plates are prolonged nearly to the front of the ventral 

 valve. 



Distrihution, 220 species. Lower Silurian — Trias. Arctic 

 America — Chili, Falkland Islands, Europe, China, Thibet, 

 Australia, Tasmania. In China these and other fossils are used 

 as medicine. 



Sub-genera. Spiriferina, D'Orbigny. S. Walcotti, PL XV., 

 Fig. 14. Shell punctate, external surface spinulose ; foramen 

 covered by a pseudo-doltidivim ; interior of ventral valve with a 

 prominent septum, rising from the adductor scar. Distrihution, 

 29 species. Carb. — Lower Oolites. Britain, France, Germany, 

 South America. 



Cyrtia, Dalman. C. exporrecta, PI. XV., Fig. 15. Shell 

 impunctate, pyramidal, beak prominent, area equiangular, 

 deltidium with a small tubular foramen. Fossil, 10 species. 

 Silurian — Trias. Europe. In C. huchii, heteroclyta, calceola, 

 &€., the shell is punctate. 



Suessia (imbricata), Eudos Beslongchamps, 1855. (Dedicated 

 to M. Suess). Shell like Sjnrifera ; texture fibrous; hinge area 

 wide as the shell ; foramen deltoid ; large valve with two 

 cardinal septa, and a prominent central septum, supporting a 

 little plate ; small valve with a tri-lobed cardinal process, and 



* Professor King attribntes tliis to mctaraorphism ; 5. Demarlii, Bench., from the 

 Devonian limestone, is punctate. (Carpenter. ) 



