200 PALMONTOLOGY 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 strise. 



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 

 fui'nishes 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. 



PLATYSTROPHIA, King. 1850. 



VIL Group of Orthis biforata, (Schlotheim) Davidson. 



Terebratulites bifokatus, Schlotheim. 



PLATE Vb, figs. 1-10. 



1820. Terebratiditen, Schlotheim. Peti'efacktenkiinde auf ihr. jetz. Standpunkt, p. 265. 



1830. Terebratiila, ton Eichwald. Nat. Skizze von Podolien, p. 202. 



1830. Porambonites (partim), Pander. Beitr. zur Geognosie Russlands, p. 96. 



1837. Atrijpa, HisiNHER. Letlifea Sueeica, p. 76. 



1840. Spinfe); voN Bdch. Ueber Delthyi-is, p. 44. 



1840. Spirifer, von Eichwald. Silurisohe Schicb ten-System von Esthland, p. 144. 



1842. Delthyvis, Emmons. Geology of N. Y. ; Rejit. Second District, p. 396. 



1842. DeWiyris, Conrad. Jour. Acad. Nat. Sci. Phil., vol. viii, p. 260. 



1843. DeJtliyrw, Hall. Geology of N. Y. ; Report Fourth District, p. 70. 



1843. Spirifer, Castelnau. Essai sur le Syst. silur. de TAmtSrique septentrionale, p. 42. 



1844. DeWi.yris, Owen. Geol. Exploration Iowa, Wisconsin, Illinois, pi. 15. 



1845. DeUhyris, de Verneoil. Geol. de la Russie, vol. ii, p. 135. 



1846. Delthyvis, McCoy. Synopsis Silur. Fossils Ireland, p. 37. 



1847. DeUhyrU. Hall, PaLiiiontdlog-y of N. Y., vol. i, p. 131. 



* Systeme Silurien, vol. v, pi. 63, figs. I, III. 



