ORDER III PROTREMATA 319 
Richthofena, Kayser (Fig. 533). These most remarkable modified Brachio- 
pods are found in the Permo-Carboniferous of China and 
India. 
Family 8. Orthidae. Woodward. 
Strophomenacea usually with large open delthyria ; deltidium 
developed only in younger growth stages. Ordovician to 
Permian. 
Orthis, Dalman (Orthambonites, 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 coneave. Cardinal areas well developed in @38% 
both valves, and usually divided by open delthyria. Hinge- Tee ee 
teeth large, and supported by well-developed dental plates. — Richthofenia Lawrenci- 
Dental sockets of dorsal valve deep, their inner walls [i Nagi Fem 
supporting short divergent crura. Median ridges fre- secaoaat ventral valve 
quently present in both valves. Vascular markings often agen on 
retained. Ordovician to Carboniferous ; particularly abundant in the Ordo- 

ol A 
i 
Wi 
s 

Fig. 534, 
_ A-C, Orthis (Schizophoria) striatula, Schloth. sp. Devonian ; Gerolstein, Eifel. A, Dorsal aspect. 3B, 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) elegantula, 
Dal. Silurian; Gottland. 1/). 
Fic. 537. 
Platystrophia lyne, Hichw. Ordovician ; Cincinnati, Ohio. 1/;. 
vician and Silurian, which have alone yielded about 
400 species. Distribution general. 
This genus has been recently divided by Hall and 
Clarke into fifteen sub-genera, as follows :—Orthis s.s., 

Fic. 536. Plectorthis, Dinorthis, Plaesiomys, Hebertella, Orthostrophia, 
Lik, eieeoeeana Platystrophia, King (Fig. 537), Heterorthis, Bilobites, 
Shell, 1/;. B, Interior of dorsal Linné (Fig. 536), Dalmanella (Fig. 535), Rhipidomella, 
Malve, i Oehlert, Schizophoria, King (Fig. 534), Orthotichia, 
Enteletes, Fischer de Waldheim, and Orthotropia. 
