189 PALJSONTOLOGY OF NEW- YORK. 



Productella arctirostrata. 



PLATE XXVI. 



Productua arctirostratut : Hall in Tenth Report on the State Cabinet, p. 177. 1867. 

 Compare Productella shumardiana of the Hamilton group, page 157 of this volume. 



Shell subovate, extremely ventricose, the proportions of length and 

 breadth variable : hinge-line usually a little less than half 'the width 

 of the shell. 



Ventral valve gibbous or ventricose : umbo very prominent and arching 

 over the hinge-line, flattened or slightly depressed in the centre, ab- 

 ruptly depressed at the sides, and extended into short ears at the car- 

 dinal extremities. 



Dorsal valve moderately concave in the upper part, abruptly deflected 

 towards the front, and following the curvature of the ventral valve. 



Surface covered by close concentric striae, with a few strong wrinkles 

 on the cars ; and these are more or less distinctly shown, extending 

 across the shell, or marking the surface of the cast. The bases of a few 

 spines are usually seen on the ears of the ventral valve, and the umbo 

 and upper part of the shell are marked by short spiniferous ridges. 

 These ridges become elongated in the lower part of the shell, and 

 are finally almost or quite continuous towards the front. The dorsal 

 valve is similarly marked; the concentric wrinkles extends across 

 the shell, and the spine bases, which are mere nodes in the upper part 

 of the valve, become elongated towards the front. 

 The casts of the dorsal valve show a bilobed cardinal process, but its 



character cannot be fully made out. The casts of the ventral valve show 



small cavities, from the removal of the dental lamellae, and there is an 



extremely narrow area visible. 



The figures 19-21, illustrate the ordinary form of the ventral valves as they occur 



in easts in sandstone. Figures IG and 17, arc of the interior, and u jirofile view of a 



dorsal valve. 

 Figures 22 and 23, arc of a ventral valve and profile of the same, which is a large and 



somewhat extravagant form. The flattening of the upper part of the valve is continued 



in a broad depression to the lower part of the shell. 



