116 
BRACHIOPODA. 
8. Argiope Buchii. 
Shell minute, transverse, subquadrate, depressed ; valves orna- 
mented with six corresponding ribs ; hinge-line wide as the 
shell, straight; area narrow; foramen small. Lon. and lat. l-^ 
line. 
Ortliis Buchii, Hagenow, 1842, Neues Jahrb. p. 544. t. 9. f. 8. 
Terebratula Buchii, Bronn, Index, p. 1231. 
Fossil. Chalk. Riigen. 
9. ? Argiope depressa. 
Shell transverse, depressed, radiately 10-ribhed ; ribs straight, 
naiTOw ; valves unequal, the ventral convex, the dorsal flat ; area 
triangular, nan-ow. Lon. 2, lat. 3 lines. 
Megathyris depressa, D’Orbigny, 1847, Ter. Cr^t. iv. p. 149. 
t. 521. f. 12-16. 
Fossil. Chalk. France. 
10. Argiope hirundo. 
Shell minute, quadi’ate, depressed, side and front deeply in- 
dented; valves with two diverging ribs; beak small; foramen 
large. Lon. and lat. 1 line. 
Orthis hirundo, Hagenow, 1842, Neues Jahrb. p. 545. t. 9. f. 9. 
Terebratula hirundo, Bronn, Index, p. 1238. 
Fossil. Chalk. Riigen. 
Fam. 2. THECIDEIDiE. 
Shell thick, punctate, rounded or oval, more or less regular, 
attached by the umbo of the ventral valve, or free : ventral valve 
(fig. 25) with the umbo rather produced and perforated by a 
minute apical foramen (/) usually closed at an early age ; hinge- 
area flat, triangular; deltidium {d) triangular, scarcely distinct 
from the area ; interior deeply concave, furnished with tnm pro- 
minent cardinal teeth {t) ; surface striated, with smooth depres- 
sions indicating the points of attachment of the adductor (a), re- 
tractor (r), and pedicel muscles (p) ; dorsal valve (fig. 24) small, 
rounded, opercular, furnished interiorly with a prominent car- 
dinal process (j) between the dental sockets ; oral processes 
united, forming a bridge over the small and deep visceral cavity 
and separating the anterior (oral) from the posterior (anal) 
orifice ; disc grooved for the reception of the loop, the grooves 
separated by branches from a central septum ; loop often un- 
symmetrical, divided into two or more lobes, united more or less 
intimately vritb the sides of the grooves. 
