140 PALAEONTOLOGY OF KENTUCKY. 



Streptorhynchus arctostriata. HALL. 



Plate XXXI., figures 31, 32 and 33. 

 Orthisina arctostriata, Hall. 13th Reg. Report 1860. 

 Streptorhynchus arctostriata, Hall. Pal. N. Y., Vol. 4, p. 711867. 



Shell of medium size, semi-circular or semi-elliptical, frequently unsymmet- 

 rical ; the proportion of length to width differs in different specimens ; hinge- 

 line straight, differing in length in different individuals, but in the average 

 about equal to the greatest width of the shell ; lateral margins meet the car- 

 dinal line usually at right-angles; they have generally a compound curve, 

 concave in the upper, and convex in the lower half. Ventral valve more or 

 less convex towards the urn bo, and sometimes in the middle, being more or 

 less flattened towards the sides and front of the shell ; beak often distorted ; 

 area straight, triangular and well defined in its margins ; inclines sometimes 

 forward, and again, in other specimens, backwards ; it is often unequal in its 

 two sides as divided by the triangular fissure, which is closed by a strong con- 

 vex deltidial plate. 



Dorsal valve depressed convex, but in some shells, partly in consequence of 

 distortion, markedly ventricose, with a narrow or linear area. Surface orna- 

 mented by sharp, close, radiating, crenulated striae, which increase mainly 

 by interpolation or interstitial addition. 



This shell is very variable in its main characters ; its beak, generally dis- 

 torted, is sometimes exactly straight; the cardinal area, unequal in most 

 specimens, is, in some individuals, fully symmetrical ; the ventral valve is, in 

 most of our shells, crushed, and consequently does not show its original form ; 

 while the dorsal valve of our specimens is, even in individuals not distorted, 

 considerably convex, instead of the flat or slightly concave dorsal valve of 

 eastern shells of this species. 



Concentric lines of growth are more or less shown on different shells. 



Formation and Locality. Found in the upper strata of our Devonian formation, which by Prof 

 Hall are considered as belonging to the Hamilton group ; it is a rare shell, and to my knowledge, so far, 

 never found undistorted. The specimen illustrated on plate 81, figures 32 and 33, represents about the 

 largest size attained by this species, at least in our rocks. 



