314 THECIDIIDHZ—RHYNCHONELLIDA. 
valve with a longitudinal septum in the middle; hinge-area 
distinct ; foramen large and angular in the young shell, gradually 
surrounded by 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. 
CRYPTACANTHIA, 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.” S. compacta, W. and St. J. Carb.; 
Towa. . 
Famity THECIDIID A. 
Shell (perforate when young?) attached by the neural valve 
-when adult. Brachia lobed, not spiral. Shell articulated by 
teeth, sockets and a cardinal’ process. 
TuHecipra, Defrance, 1828. 
Etym.— Thekidion, a small pouch. 
Syn.—Bactrynium, Emmrich. Pterophloios, Gumbel. The- 
cidium, Sowb. 
Distr.—2 sp. Mediterranean, W. Indies. Fossil, 34 sp. 
Trias—; Europe. TI’. papillata, Schloth. (exxxvi, 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 
united, 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. 
Famity RHYNCHONELLIDA. 
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- 
