64 
PALAEONTOLOGY OF NEW-YORK. 
GENUS StREPTORIIYACIIUS (King). 
The Genus Streptorhynchus 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 
having 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 Streptorhynchus ; and among the Lower Silu¬ 
rian forms, Strophomena recta, S. defleda, S. filitexta* 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 strise, which 
are crossed by fine concentric lines; and in some forms the stronger 
striae are distant, with finer radiating and concentric striae Gancellating 
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. 
* See Sixteenth Report on the State Cabinet, 1863., 
