BRACHIOPODA 



177 



Orthothetes . Shell plano-convex or biconvex, sometimes con- 

 cavo-convex in old age, ornamented with fine radiating ribs and 

 concentric lines ; hinge-line equal to or longer than the width of the 

 shell. Ventral area prominent, often high, the two sides sometimes 

 unequal, umbo often irregular; delthyriurn closed by a convex pseudo- 

 deltidium which is rarely perforated in the adult ; muscular impres- 

 sions fan-shaped ; teeth not supported by dental plates. Dorsal valve 

 with narrow area ; the cardinal process united to internal side plates. 

 Silurian to Carboniferous. Ex. 0. crenistria, Carboniferous. 



Orthis (fig. 80). Outline sub-circular or quadrate. Both 

 valves convex. Surface radially ribbed or striated. Hinge-line 

 straight, sometimes equal to the width of the shell, but generally 

 shorter. An area on each valve, each divided by an open delthyriurn. 

 In the ventral valve two large teeth, supported by dental plates. 

 Four muscular impressions in the dorsal valve. Two long, narrow 



-V 



A B 



Fig. 80. Orthis (Schizophoria) striatula, Devonian. A, interior of dorsal 

 valve ; B*, ventral valve. c, curved brachial processes (crura) ; 

 v, genital impressions; h, area with delthyriurn; t, teeth ; a, adductors; 

 d, divaricators. (From Woodward.) Natural size. 



impressions (d) with two smaller ones (a) between them in the 

 ventral. Cambrian to Carboniferous. Ex. 0. calligramma, Ordo- 

 vician ; 0. flabellulum, Bala Beds ; 0. resupinata, Carboniferous 

 Limestone. Orthis includes a large number of species and has been 

 divided into several sub-genera, some of which are Orthis (restricted), 

 Plati/strophia, Bilobites, Dalmanella, Schizophoria, Rhipidomella. 



w. p. 



12 



