24 PALAEONTOLOGY OF THE UPPER MISSOURI. 



Genus IIEMIPRONITES, PANDER. 



Synon.Terebratuhtes (sp.), Senior. Akad. Munch., VI, 1816, 28. 

 Hemipronites, PAKDEB, Beitr. zur Geol. Russ. 1830, 75. 

 Klitambonites (part), PAXDER, 1830, Ib., 70. 

 Gonambonites, PANDER, Ib., 1830, 77. 



Spirifer (sp.), PHILLIPS, Geol. York. 1836, II (not SOWERBT, 1815). 

 Lepttena (sp.), J. DE C. SOWEKBY, Min. Conch. 1840 (not DALMAN, 1828). 

 Orthit (sp.), POETLOCK, Kept. Geol. Lond. 1843, 456. KONINCK. An. Foss. Belg. 1843, 222. HALL, Geol. 



Kept. I wa, I part ii, 1858, 640 and 713 (not DALMAN 1828). 

 Orthisina (part), D'ORBIONY, Compt. Rend. XXV, 1847, 267. DAVIDSON, Brit. Foss. Brach. I, Gen. Introdnc. 



1854, 104. Orthisina (sp.), SWALLOW, Trans. St. Louis Acad. Sci. 1, 1858, 219. MEEK & HAYDEH, Proceed. 



Acad. Nat. Sci. Phila. Dec. 1858, 260. HALL, Thirteenth Kept. Regents University N. Y. 1860 (not 



D'OKBIOHT, 1849). 



Hemiprionites, AGASSIZ, Nom. Zool. Mol. 1846, 41. 



Streptorhynchus, KING, Permian Foss. England, 1850, 107. DAVIDSON, Brit. Permian Brachiopoda, 1857, 

 29. HALL, Thirteenth Report Regents University N. Y., 1860, 112; also Sixteenth Kept. Dec. 

 1863, 61. 



Etym. Kpi, half; {, prominence. 

 Type. Hemipronites tumidus, PANDEB. 



Shell varying from truncatoorbicular to semicircular, or orbicular subquadrate, 

 more or less convex, the inequality of the valves varying greatly with the species ; 

 surface marked with radiating, generally straight striae, and sometimes with 

 rounded radiating plications. Hinge usually shorter than the greatest breadth of 

 the valves ; provided in the ventral or larger valve with two teeth, situated one 

 on each side of the mesial fissure, and fitting into corresponding sockets in the 

 other valve. Structure probably always impunctate. 



Ventral valve with its beak more prominent than that of the dorsal, often bent 

 or twisted, but not regularly incurved; area generally high, sometimes extremely 

 so, its mesial fissure always closed, in adult shells, by a convex pscudo-deltidium. 

 Hinge teeth supported within by two dental plates, which converge under the area 

 towards the beak. Scars of cardinal and adductor muscles occupying about one- 

 third to one-half the length of the valve (between the beak and the middle), and 

 forming two elongated oval impressions, more or less deeply excavated, one on each 

 side of a mesial ridge or septum. 



Dorsal valve generally with its beak compressed and projecting little beyond 

 the cardinal margin ; area usually very narrow or rudimentary. Cardinal process 

 large, prominent, and bifid; either slightly convex or concave on the inner side, 

 with each division more or less grooved or emarginatc at the extremity of the 

 outer side ; on each side of, and connecting with this, are the well-developed socket 

 plates. At the bottom of the valve the quadruple scars of adductor muscles occupy 

 about one-third the length of the valve, being arranged in pairs on each side of a 

 short mesial ridge. 



The shells belonging to this group, although usually regarded as constituting 

 a section or subgenus of Orthis, present sufficiently marked differences to rank 

 as a distinct genus. In the first place they differ in having the foramen always 



