ORDER III 



]'i;oTi;i-:.MATA 



319 



JUrhthofenia, Kayser (Fig. 533). These most remarkable modified Brachio- 

 pods are found in the Permo-Carboniferous of China and 



linlia. 



Family 8. Orthidae. Woodward. 



Strophomenacea usually with large open delthyria ; deltidium 

 developed only in younger growth stages. Ordovician to 

 Permian. 



Orthis, Dalman (Orllminlxmites, Pander), (Figs. 534-536). 

 Shell sub-quadrate to sub-circular in outline; surface 

 covered with radiating striae or costae. Valves more or 

 less convex ; the small or dorsal valves sometimes nearly 

 flat or slightly concave. Cardinal areas well developed in 

 both valves, and usually divided by open delthyria. Hinge- 

 teeth large, and supported by well-developed dental plates. 

 Dental sockets of dorsal valve deep, their inner walls 

 supporting short divergent crura. Median ridges fre- 

 quently present in both valves. Vascular markings often 

 retained. Ordovician to Carboniferous ; particularly abundant in the Ordo- 



Fio. 533. 



Richthofenia Lawrenci- 

 an,Waagen. Penno-Car- 

 bon.; Salt Range. Y.-rtiral 

 section of ventral valve 

 (after Waagen). 



FIG. 534. 



A~C, Orthis (Schizophoria) striatula, Schloth. sp. Devonian ; Gerolstein, Eifel. A, Dorsal aspect. E, Interior 

 of dorsal valve. C, Interior of ventral valve. D, Orthis vulvaria, Schloth. Spiriferensandstein ; Niederlahn- 

 stein, Nassau. (All figures of the natural size.) 



FIG. 535. 



Orthis (Dalmanella) elegantuta, 

 Dalni. Silurian ; Gottlaud. i/j. 



FIG. 537. 

 riutystrophia lynx, Eichw. Ordoviciau ; Cincinnati, Ohio. 1/1- 



vician and Silurian, which have alone yielded about 

 ^HR /, ^0 species. Distribution general. 



ft vmy^ijjij This genus has been recentlv divided ^y Hal1 and 



^ ^^ Clarke into fifteen sub-genera, as follows : Orthis s.s., 



Plectorthis, Dinorthis, Plaesiomys, Hebertella, Orthostrophia, 

 Platystrophia, King (Fig. 537), Heterorthis, Bilobites, 

 Linne (Fig. 536), Dalmanella (Fig. 535), Rhipidomcll', 

 Oehlert, Schizophoria, King (Fig. 534), Orthotichia, 

 Enteletes, Fischer de Waldheim, and Orthotropia. 



FIG. 536. 



Orthis (BiloUtes) Uloba, 

 Lin. Silurian ; Gottland. A, 

 Shell, 1/1. B, Interior of dorsal 

 valve, 2/1. 



