334 



MOLLUSCOIDEA BRACHIOPODA 



SUB-KINGDOM V 



with the complete jugum decidedly posterior in position. Ordovician arid 

 Silurian ; North America. 



Atrypina, Hall and Clarke. Primitive Atrypae with few plications, and 

 but three or four volutions in each spiral. Jugum as in Atrypa, but continuous. 



Silurian to Devonian ; 

 North America and 

 Europe. 



Glassia, Davidson 

 (Fig. 570). Small smooth 

 shells, with the apices of 

 the laterally compressed 

 spirals situated at the 

 centre of the brachial 

 cavity. Jugum similar 

 to that of Atrypina. Or- 

 5/2 (after David- dovician to Devonian ; 



Europe. 



Dayia, Davidson (Fig. 571). Small smooth shells, with the jugum situated 

 anteriorly, and drawn out posteriorly into a simple short process. Spiralia 

 laterally directed. Silurian ; Europe. 



Atrypa, Dalman (Cleiothyris, Phill. ; Spirigerina, d'Orb.), (Fig. 572). Shell 

 radially plicated, usually with lamellar expansions or hollow spines. Spirals 

 introverted, dorso-medially directed. Jugum extremely posterior in position, 



FIG. 570. 



Glassia obovata, Sow. sp. 

 Silurian ; Wenlock, England. 

 Interior of dorsal valve, 3 /i 

 (after Davidson). 



FIG. 571. 



Dayia navicula, Sow. sp. 

 Ludlow, Shropshire, 

 son). 



Silurian 



FIG. 572. 



Atrypa reticularis, Lin. sp. Middle Devonian; Gerolstein, Eifel. A, Dorsal aspect of adult specimen. 

 B, Ventral aspect and profile of young shell. C, Interior of dorsal valve, showing spiralia, crura, and jugum. 

 D, Ventral valve, showing muscular and vascular impressions (, Adductors ; c, Diductors ; d, Deltidial plates ; 

 o, Ovaries ; p, Pedicle muscle). 



and complete in young stages ; but mesially absorbed at maturity. Widely 

 distributed in the Silurian and Devonian. 



Gruenewaldtia, Tschernyschew. Atrtjpa-\ike> shells, but with the relative 

 convexity of the valves reversed. Devonian ; Eussia. 



1 Karpinskya, Tschernyschew. Elongate, Atrypa-\ike shells, with a median 

 dorsal septum. Devonian ; Eussia. 



1 Clintonella, Hall and Clarke. Silurian ; North America. 



Family 2. Spiriferidae. King. 



Spiriferacea with the crwa directly continuous with the bases of the primary 

 lamellae, which are situated between the laterally directed spiralia. Jugum simple, 

 complete, or incomplete. Silurian to Jurassic. 



