328 8TR0PH0MENID^. 



trunks in the dorsal valve, two in the ventral, the external 

 branches turned outwards and backwards, inclosing wide ova- 

 rian spaces. Indications have been observed, in several genera, 

 of horizontally coiled spiral arms; the space between the 

 valves is often very small. The shell-structure is punctate, 

 except in a few instances, where the original texture is probably 

 obliterated. 



Orthis, Dalman, 1827. 



Etym. — OrtJios^ straight. 



Syn. — Orthambonites, Pander, 1830. Schizophoria, King, 1850. 



Distr. — Fossil, 300 sp. Lower Silurian — Carb. ; Arctic Am., 

 United States, South America, Falkland Islands, Europe, Thibet. 

 0. striatida, Schloth. (cxxxviii, 3t-39). 



Shell transversely oblong, radiately striated or plaited, bicon- 

 vex, hinge-line narrower than the shell, cardinal process simple, 

 brachial processes tooth-like, prominent and curved. 



BiLOBiTES, Linn., 1775. (Dicoelosia, King, 1850.) Ventral 

 margin deepl_y cut out in the middle, forming two lobes in each 

 valve ; two strong curved crural processes from the hinge-margin 

 of the smaller valve. Silurian ; Gotland and U. S. 0. biloba, 

 Linn. 



PLATYSTROPHiA, King, 1850. Shell somewhat transverse, 

 nearly equivalve, with a long, straight hinge-line ; both valves 

 very convex, radially ribbed, with an area and trigonal deltidial 

 opening; beaks curved, approaching ; large valve with a deep 

 median sinus. Silurian, Carboniferous. 0. lynx, Eichw, 

 (cxxxviii, 40). 



ENTELETES, Fischcr, 1830. (Choristites, Fischer [part], 1825.) 

 Like Flat3"strophia,but the hinge-margin short ; surface coarsely 

 wrinkled, and finel}^ radially sculptured. 0. Lamarcki^ Fischer. 

 Carboniferous ; Russia. 



MYSTROPHORA, Kayser, 1871. Shell like Orthis, but the small 

 valve with a very high median septum, which reaches to the 

 other valve, and so divides the closed shell into two chambers ; 

 the two tooth-plates are united to the septum. Silurian, Devo- 

 nian. 0. Leioifiii, Davids. Davidson makes this a synonym of 

 Skenidium, Hall. 



Streptorhynchus, King, 1850. 



Etym. — Stre'pto, I bend or twist ; rJiynchos, a beak. 



Syn. — Hipparionix, Vanuxem, 1842. Orthotetes (Evans), 

 Fisch. (part), 1829. 



Distr. — 6 sp. Sil. — Perm. ; Europe, Asia, America, and 

 Australia. S. Devonica, d'Orb. 



Shell inequivalved, convex or concavo-convex, externally 

 striated ; hinge-line rather shorter than the width of the shell ; 

 dorsal valve semicircular, with a small narrow area. Ventral 



