64 PALEONTOLOGY OF NEW-YORK. 



GEXIS SXREPTOKIIYXCIirS (King). 



The Genus Streptorhynchcs was published in 1850 by Prof. King, to 

 include some peculiar formS of strophomenoid shells ; one conspicuous 

 character of which is that the umbone of the larger valve is irregularly 

 twisted. The genus possesses features somewhat intermediate to Orthis 

 and Strophomena ; and for want of a full understanding of these'charac- 

 ters, the species have been referred, first by Davidson, and after him by 

 other authors, to the Genus Orthisina. Regarding the twisted or distorted 

 beak as one of the essential features of the genus, there are regular forms 

 liaving internal characters corresponding with the typical species of the 

 genus, which on the one side have been forced among the Orthis, while 

 others have been placed among Strophomena. 



As at present constituted, the genus will include such forms as I have 

 described in the third volume (Pal. New- York) under the names Stropho- 

 mena woolworthana and S. radiata, which are probably identical species. 

 Going back to the Niagara form S. subplana, we find that it also may be 

 included under the Genus Streptorhynchds ; and among the Lower Silu- 

 rian forms, Strophomena recta, S. defleda, S. jilitexta* and some others, may 

 be included in the same group ; these latter showing the lowest indica- 

 tions of those features of the hinge-structure which are more strongly 

 developed in the species of later geological periods. 



The shells of this genus are semicircular or semielliptical, concavo- 

 convex or plano-convex, and sometimes with both valves convex : they 

 are externally striated with rounded bifurcating threadlike stria), which 

 are crossed by fine concentric lines; and in some forms the stronger 

 striae are distant, with finer radiating and concentric striae cancellatiug 

 the intermediate spaces. The ventral beak is sometimes produced and 

 bent or twisted, and the fissure beneath the beak is closed or partially 

 closed by a solid deltidium, while the area is subject to great variation. 

 A narrow area often exists on the dorsal valve, but this is not a constant 

 character. 



• Sco Sixteenth Report on the State Cabinet, 1863. 



