DESCRIPTIONS OF SPECIES 365 



limestone in nearly all its exposures in Erie county. Most of the indi- 

 viduals lie parallel to the bedding plane, having fallen before they were 

 buried. No sj)ecimens are known from localities outside of the county. 



ORTHOTHETES HYDRAULICUS {WHITFIELD) 



(Plate 22, figures la-c.) 



1882. Streptorhynchus hydraulicum Whitfield. Annals of the New York Academy of 

 Sciences, volume ii, page 193 ; volume v, 1891, page 508, plate 5, figures 1-3. 



1893. Streptorhynchus hydraulicum Whitfield. Report of the Geological Survey of 

 Ohio, volume vii, page 410, plate I, figures 1-3. 



This species, originally described from Bellville and Greenfield, Ohio, 

 is abundant in the Manlius limestone of Erie county. The shell is 

 rarely preserved, but the molds are often very perfect, allowing sharp 

 and clear gutta-percha casts to be taken. 



The pedicle valve has a slightly elevated beak, with a low, triangular 

 cardinal area, which is flat and transversely striate; delthyrium mod- 

 erate, covered in great part by a strong convex deltidium. The cardi- 

 nal teeth are prominent and supported by two short and narrow dental 

 plates, which have the same angle of divergence as the sides of the 

 delthyrium. The cardinal extremities are obtuse, the hinge line being- 

 shorter than the greatest width of the shell, while the front is uniformly 

 rounded. 



The brachial valve has a very narrow hinge area, which is erect, 

 making a moderately obtuse angle with the hinge area of the pedicle 

 valve. A strong band-like chilidium covers the median fissure. Be- 

 tween it and the deltidium there is a narrow open space, through which 

 can be seen the cardinal process, which appears bilobed ; surface of both 

 valves* marked with strong, rounded, but sharply defined radiating 

 striae, which curve slightly upward on the lateral margins near the car- 

 dinal area. The strongest of these reach close up on to the beak. 

 Passing forward, new striae appear between them as soon as they have 

 separated by more than their own width. Additional sets of stria? 

 appear as the shell increases in size, these having been observed up to 

 the fifth generation. The stride are cancellated by uniform, close, fine, 

 and regular concentric lines, which are most prominent on the stria?. 



A mold of the interior shows the striae quite strongly, and even the 

 cancellations are visible. It is not improbable, however, that after the 

 solution of the shell, the two molds becoming closely appressed, the 

 stronger external features were impressed upon the weaker internal, 

 thus accounting for the markings, which would otherwise indicate a 

 shell of great tenuity. The muscular impressions have not been re- 

 tained in the molds. 



