314 MOLLUSCOIDEA—BRACHIOPODA SUB-KINGDOM V 
Leptaenisca, Beecher. 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. 
aoe Davidsonia, Bouchard (Fig. 520). Thick Leptaenisca- 
Davidsonia Bouchar- 4+ : : : 
diana, de Kon. Devonian; like shells, with spiral markings of the fleshy arms strongly 
emake 2. impressed on both valves. Devonian; England, Belgium, 
and Russia. 
Strophomena, Blainville. Shells like Rafinesquina, 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. Ze 
Orthothetes, Fischer (Figs. 521, 522). ‘Y 
Much like Strophomena. Shell plano-convex & 
or biconvex, but becoming concayo-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 Fic. 521. 
forming a vertical sub-crescentic process. — Orthothetes wmbraculwm, Schloth. sp. De- 
Muscular impressions flabelliform. Silurian YON) Gerelstem, Bifel. Natural size. 
to Carboniferous ; North and South America, Europe, and India. 
Hipparionyz, Vanuxem. Like Orthothetes, but with the muscular areas 
much larger and no dorsal cardinal area. Lower Devonian ; North America. 




Fic. 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 
