116 brachiopoda. 



8. Argiope Buchii. 



Shell minute, transverse, subquadrate, depressed ; valves orna- 

 mented with six corresponding ribs ; hinge-line wide as the 

 shell, straight; ai-ea narrow; foramen small. Lon. and lat. 1^ 

 line. 



Orthis 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-ribbed ; ribs straight, 

 narrow ; valves unequal, the ventral convex, the dorsal fiat ; area 

 triangular, narrow. Lon. 2, lat. 3 lines. 



MegathNTis depressa, D'Orbigny, 1847, Ter. Cret. 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 ibverging ribs; beak small; foramen 

 large. Lon. and lat. 1 line. 



Orthis hirundo, Hagenow, 1842, Neues Jahrb. p. 545. t. 9. f. 9. 

 Terebratula hii-undo, Bronn, Index, p. 1238. 

 Fossil. Chalk. Rugen. 



*n^ 



Fam. 2. THECIDEID^. 



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 

 fi'om the area ; interior deeply concave, furnished with two 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 (/;) ; 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 with the sides of the grooves. 



