314 THECIDIID-^ — RHYNCHONELLIDiE. 



valve with a longitudinal septuna in the middle ; hinge-area 

 distinct ; foramen large and angular in the young shell, gradvially 

 surrounded b}^ the deltidium, and rendered small and oval in the 

 adult; deltidium composed of three elements ; teeth prominent ; 

 dorsal valve depressed, cardinal process very prominent, some- 

 times touching the opposite valve, its extremity forked to receive 

 the ventral septum ; hinge-plate supporting a shelly loop, after 

 the manner of Argiope. 



CRYPTAOANTHiA, White and St. John, 1868. " The loop seems 

 to be essentially like that of Waldheimia in form, but the crura 

 of the loop appear to be joined, * * * and the loop-band is 

 armed with numerous spines which point outward toward the 

 shell (?) in all directions." 8. compacta, W. and St. J. Carb. ; 

 Iowa. 



Family THECIDIID^. 



Shell (perforate when young ?) attached by the neural valve 

 when adult. Brachia lobed, not spiral. Shell articulated by 

 teeth, sockets and a cardinal process, 



Thecidia, Defrance, 1828. 



Etym. — Thekidion, a small pouch. 



Syn. — Bactrynium, Emmrich. Pterophloios, Gumbel. The- 

 • cidium, Sowb. 



Dist7\ — 2 sp. Mediterranean, W. Indies. Fossil, 34 sp. 

 Trias — ; Europe. J^.jJap^7^ai;a, Schloth. (cxxxvi, 57-59). 



Shell small, thick, punctate, attached by the beak , hinge-area 

 flat; deltidium triangular, indistinct; dorsal valve rounded, 

 depressed: interior with a broad granulated margin; cardinal 

 process prominent, between the dental sockets ; oral processes 

 imited, forming a bridge over the small and deep visceral cavity ; 

 disk grooved for the reception of the loop, the grooves separated 

 by branches from a central septum ; loop often unsymmetrical, 

 lobed, and united more or less intimately with the sides of the 

 grooves ; ventral valve deeply excavated, hinge-teeth prominent; 

 cavities for the adductor and pedicel muscles small; disk occu- 

 pied by two large, smooth impressions of the cardinal muscles, 

 •bordered by a vascular line. 



Animal with elongated oral arms, folded on themselves and 

 fringed with long cirri; mantle extending to the margin of the 

 valves and closely adherent ; epidermis distinct. 



Family RHYNCHONELLID^. 



Shell impunctate, oblong, or trigonal, beaked ; hinge-line curved; 

 no area; valves articulated, convex, often sharply plaited; for- 

 amen beneath the beak, usually completed by a deltidium, some- 

 times concealed ; hinge-teeth supported by dental plates; hinge- 



