314 



MOLLUSCOIDEA BRACHIOPODA 



SUB-KINGDOM V 



FIG. 520. 



Bouchar- 

 Devonian: 



Leptaenisca, Beech er. Ventrally cemented shells having some of the char- 

 acters of Plectambonites. Markings of the fleshy arms are 

 retained on the dorsal shell. Silurian ; North America. 



Christiania, Hall and Clarke. Differs from Leptaenisca 

 in having prominent longitudinal ridges instead of spiral 

 markings on the dorsal interior. Basal Devonian ; North 

 America, England, and Russia. 



Davidsonia, Bouchard (Fig. 520). Thick Leptaenisca- 

 like shells, with spiral markings of the fleshy arms strongly 

 impressed on both valves. Devonian ; England, Belgium, 

 and Russia. 

 Strophomena, Blainville. Shells like Piafinesquina, but resupinate, or with the 



relative convexity of valves reversed, and the ventral muscular area sharply 



limited by an elevated margin. Ordovician ; 



America and Europe. 



Orthothetes, Fischer (Figs. 521, 522). 



Much like Strophomena. Shell plano-convex 



or biconvex, but becoming concavo-convex 



with age. Surface covered with radiat- 

 ing striae, which are convoluted by sharp 



concentric lines. Hinge - line as long or 



longer than the greatest width of the shell. 



Cardinal area of ventral valve prominently 



developed; dorsal area narrow. Cardinal 



process united to crural plates, the whole 



forming a vertical sub-crescentic process. 



Muscular impressions flabelliform. Silurian 



to Carboniferous ; North and South America, Europe, and India. 



Hipparionyx, Vanuxem. Like Orthothetes, but with the muscular areas 



much larger and no dorsal cardinal area. Lower Devonian ; North America. 



FIG. 521. 



Orthothetes umbmculum, Schloth. sp. De- 

 vonian ; Gerolstein, Bifel. Natural size. 



FIG. 522. 



Orthothetes crenistria, Phill. Carboniferous Limestone ; Wexford. A, Muscular portion of ventral valve. 

 B, Interior of dorsal valve. (A, A', Adductors. R, Diductors ; j, Cardinal process ; d, Dental sockets), (after 

 Davidson). 



Kayserella, Hall and Clarke. Small Orthothetes-like shells, with a very high 

 dorsal median septum. Devonian ; Germany. 



Derbya, Waagen. Like Orthothetes, with a high median septum in the ventral 



