BfiACHIOPOTIA. 



373 



a broad 4-partite Mnge-plate, with, processes from the outer 

 angles of the dental sockets ; crura of the spires united by a 



transverse band supporting a small process. 

 Upper Lias, Normandy. 



Fossil, 2 species. 



Atkfris, M'Coy. 



Etymology i a, without, thuris, a door* (i.e. deltidium). 



Synonyms, Spirigera, D'Orbigny. Cleiothyris, King (not 

 Phil.). 



Types, A. concentrica, Buch. A. Eoissyi, Pigs. 169, 170. 

 A. lamellosa, PL XY., Fig. 16. 



Shell impunctate, transversely oval, or sub-orbicular, bi« 

 convex, smooth, or ornamented with squamose lines of growth, 

 sometimes developed into wing-like expansions (Pig. 170t) ', 



Fig. 169. Interior of dorsal valve. 



Fig. 170. Specimen with fringe. 



hinge-line curved, area obsolete, foramen round, truncating the 

 beak, deltidium obsolete ; hinge -plate of dorsal valve with four 

 muscular cavities, perforated by a small round foramen, and 

 supporting a small complicated loop (?) between the spires ; 

 spires directed outwards, crura united by a prominent oral 

 loop. 



The foramen in the hinge-plate occupies the situation of the 

 notch through which the intestine passes in the recent Bhyn- 

 cJionellce; in A. coricentrica a, slender curved tube is sometimes 

 attached to the foramen, beneath the hinge-plate. A. tumida 

 has the hinge-plate merely grooved, and the byssal foramen is 

 angular. 



Fossil, about 70 species. Silurian — Lias. North and South, 

 America and Europe. 



* Sometimes employed, incorrectly, in the sense of a 6.ooT-v)ay or for men. 

 t The spurious genus Actinoconchus (M'Coy) was founded on this character ; similai' 

 expansions are formed by species of Atrypa, Camarophoria, and Producta. 



