;72 



MANUAL OF THE MOLLUSCA. 



Shell impunctate,* transversely oval or elongated, tri-lobed, 

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

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

 foramen angular, open in tbe young, afterwards progressively 



Dorsal valve. 



Fig. 168. 



Ventral valve. 



closed ; ventral valve with prominent binge-teetb, and a central 

 muscular scar, consisting of tbe single adductor flanked by two 

 cardinal impressions ; dorsal valve with a small cardinal process, 

 a divided binge-plate, and two conical spires directed outwards 

 and nearly filling tbe cavity of tbe sbell ; crura united by an 

 oral loop. Tbe sbell and spires are sometimes silicified in lime- 

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

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

 valve. 



Bistrilution, 220 species. Lower Silurian — Trias. Arctic 

 America — Cbili, Falkland Islands, Europe, Cbina, Tbibet, 

 Australia, Tasmania. In Cbina tbese and otber fossils are used 

 as medicine. 



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

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

 covered by a pseudo-deltidium ; interior of ventral valve witb a 

 prominent septum, rising from tbe adductor scar. Distribution, 

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

 Soutb America. 



Cyrtia, Dalman. 0. exporrecta, PL XV., Fig. 15. Shell 

 impunctate, pyramidal, beak prominent, area equiangular, 

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

 Silurian — Trias. Europe. In G. buchii, heteroclyta, calceola, 

 &c., tbe sbell is punctate. 



Suessia (imbricata), Eudes Deslongcbamps, 1855. (Dedicated 

 to M. Suess). Shell ]ike Spirifera; texture fibrous; binge area 

 wide as tbe sbell ; foramen deltoid ; large valve witb two 

 cardinal septa, and a prominent central septum, supporting a 

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



* Professor King attributes this to metamorpliism ; 5. Deniarlii, Bonch., from the 

 Devonian limestone, is ijunctate. (Carpenter.) 



