200 
PALAEONTOLOGY OF NEW YORK. 
lateral margins of the anterior adductors. The outer member of each pair 
curves quite abruptly toward the cardinal angle, while the inner members curve 
outward and then inward, all ramifying as they approach the margins of the 
shell. The umbo-lateral spaces enclosed by the curvature of these sinuses in 
both valves, and the central space in the brachial valve, are covered with 
linear dendritic ovarian striae. 
The only well known representative of this group is the Orthostrophia stro- 
phomenoides, of the Lower Helderberg fauna, with which the Orthis Halli , of 
Safford, described from the Lower Helderberg of Perry county, Tennessee, is 
probably synonymous. 
This species is the latest known representative of the impunctate orthids, and 
furnishes an interesting example of the persistence of an external form charac¬ 
terizing earlier faunas, accompanied by radical modifications of the interior. 
The Niagara species, Orthis fasciata, Hall, and the two species, Orthis mimica 
and 0. socialis* described by Barrande, from the Etage D, have interiors very 
similar in structure to that of Orthostrophia. These are, however, small, 
non-resupinate shells. 
PL AT YSTROPHI A, King. 1850. 
VII. Group of Orthis biforata, (Schlotheim) Davidson. 
Terebratulites biforatus, Schlotheim. 
PLATE Vb, PIGS. 1-10. 
1820. Terebratulites, Schlotheim. Petrefacktenkunde auf ihv. jetz. Standpunkt, p. 265. 
1830. Terebratula, von Eichwald. Nat. Skizze von Podolien, p. 202. 
1830. Porambonites (partim ), Pander. Beitr. zur Geognosie Russlands, p. 96. 
1837. Atrypa, Hisinger. Lethaea Suecica, p. 76. 
1840. Spirifer, von Bdch. Ueber Delthyris, p. 44. 
1840. Spirifer, von Eichwald. Silurische Schichten-System von Esthland, p. 144. 
1842. Delthyris, Emmons. Geology of N. Y. ; Rept. Second District, p. 396. 
1842. Delthyris, Conrad. Jour. Acad. Nat. Sci. Phil., vol. viii, p. 260. 
1843. Delthyris, Hall. Geology of N. Y. ; Report Fourth District, p. 70. 
1843. Spirifer, Castelnau. Essai sur le Syst. silur. de l’Amdrique septentrionale, p. 42. 
1844. Delthyris, Owen. Geol. Exploration Iowa, Wisconsin, Illinois, pi. 15. 
1845. Delthyris, de Vernedil. Geol. de la Russie, vol. ii, p. 135. 
1846. Delthyris, McCoy. Synopsis Silur. Fossils Ireland, p. 37. 
1847. Delthyris, Hall. Palaeontology of N. Y., vol. i, p. 131. 
Systeme Silurien, vol. v, pi. 63, figs. I, III. 
