BRACHIOrOTiA. 



373 



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

 angles of the dental sockets ; crui-a of the spires united by a 

 transverse band supporting a small process. Fossil, 2 species. 

 Upper Lias, Normandy. 



ATHmiis, M'Coy. 



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



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

 Phil.). 



Types, A. concentrica, Buch. A. Eoissyi, Figs. 169, 1*70. 

 A. lamellosa, PI. XY., Fig. 16. 



Shell impunctate, transyersely oval, or sub-orbicular, bi- 

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

 sometimes developed into wing-like expansions (Fig. ITOf); 



\..^ ^ 



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 spii-es ; 

 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 Rhyn^ 

 chonellce ; in A. concentrica a slender curved tube is sometimes 

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

 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 door-way or for men. 

 t The spurious genus Actinoconckiis Chl'Coy) was founded on tliis character ; similar 

 expansions are formed by species of Atrypa, Camarophoria, and l^oducta. 



