CLASS 11 BRACHIOPODA 385 



lines. Muscular area of ventral valve consisting of two broad flabellate 

 diductor scars enclosing an elongate adductor. In the dorsal valve, the 

 bilobed cardinal process is low ; the posterior arboi-escent adductor scars well 

 defined. Vascular and ovarian markings often well indicated. Ordovician 

 and basal Silurian ; North America and Europe. 



Stropheodonta Hall. Shells very much like Eafinesquina, but with the 

 cardinal margins finely denticulate and the deltidium fiat or not discernible. 

 Silurian and Devonian ; North America and Europe. 



Leptostrophia Hall and Clarke ; Douvillina Qilhlert ; and Brachyprion Shaler 

 are subgenera of Stropheodonta. Silurian and Devonian. 



Pholidostrophia Hall and Clarke. Smooth or squamose, nacreous small 

 Stropheodontas. Devonian ; North America and Europe. 



Strophonella Hall (Amphistrophia Hall and Clarke). Resupinate Stropheo- 

 dontas. Silurian and Devonian ; North America and Europe. 



Gaspesia Clarke. Aberrant Strophomenid recalling coarsely plicate Orthids. 

 May, however, be a Pelecypod. Lower Devonian ; Gaspe, Canada. 



Subfamily B. Tropidoleptinae Schuchert. 



Aberrant Strophomenidae with two very long slender crura that unite vnth a high 

 vertical dorsal septum. The family is sometimes regarded as better placed among the 

 Terebratellids of the Terebratulacea. 



Tropidoleptus Hall. Plano-convex, plicated shells with a long, straight and 

 narrow cardinal area. Teeth and dental sockets corrugated on their outer 

 surfaces. Devonian ; America, Europe and South Africa. 



Subfamily C. Davidsoniinae King. 



Small specialised Strophomenids, derived out of Bafinesquininae and devoid of a 

 pedicle, being cemented by the ventral valve to foreign objects. 



Christiania Hall and Clarke. Differs from Leptaenisca in having prominent 

 longitudinal ridges instead of spiral markings on the dorsal interior. Ordo- 

 vician ; North America, England and Russia. 



Leptaenisca Beecher. Ventrally cemented shells having 

 some of the characters of Plectambonites. Markings of 

 the fleshy arms are retained on the dorsal shell. Silurian 

 and Lower Devonian ; North America. 



Davidsonia Bouchard (Fig. 569). Thick Leptaenisca-\ike ^^ 



shells, with spiral markings of the fleshy arms strongly ^ .'. \ , , 

 impressed on both valves. Devonian ; England, Belgium iana cie Kon. Devonian ; 



^ -p . ' o o gj^gj_ Ventral valve with 



and JXUSSia. spiral markings, 2/j. 



Subfamily D. Orthotetinab Waagen. 



Strophomenids with the ventral valve convex during early growth, becoming sub- 

 sequently concave, or the reverse of the order in the Rafinesquininae. In the later 

 forms both valves tend to be convex. 



(f) Orthidium Hall and Clarke. Basal Ordovician ; North America. 



VOL. I 2 C 



