180 PALEONTOLOGY OF NEW- YORK. 



Pro<Iuctella costatula (n. 8.). 



PLATE XXVI. 

 Shell subovoid, more or less elongate and often extremely arcuate : 



hinge-line equalling or less than the width of the shell. 

 The ventral valve varies from a semielliptical moderately gibbous form 

 in the young, to extremely arcuate in the older shell. The young 

 shells are wider than long, and the older are much longer than wide, 

 but varying greatly in proportions of length and breadth, some of them 

 being extremely narrow. 

 The surface of the ventral valve is wrinkled on the ears, and closely 

 striate concentrically on the body of the shell. The umbo and upper 

 part of the valve are marked by numerous and regularly disposed elon- 

 gate spiniferous nodes, which support slender spines. These nodes 

 gradually become more elongated, and, below the middle, the surface 

 is marked by continuous slender costae, from which, at intervals, rise 

 slender spines. In the young shells, and the upper half of the older 

 ones, the costae are not present, and a few small but extremely arcuate 

 forms scarcely preserve this marking on the cast ; but in the larger 

 specimens, the casts exhibit the costae in a very marked degree. The 

 casts of the ventral valve are finely puncto-striate. 

 The dorsal valves which we find associated with the ventral valves 

 described, have been deeply concave ; in the upper part they are 

 moderately concave, and more abruptly deflected towards the front. The 

 interior surface only is known, and this presents a short bilobed cardinal 

 process and short slender median septum. The cardinal line is scarcely 

 equal to the greatest width of the shell, and is terminated in short small 

 ears. There are three, four, or five somewhat strong wrinkles on the car- 

 dinal margin, which scarcely reach the centre of the shell. The entire 

 inner Burface is finely papillose-striate, and shows concentric striae ; 

 while it is thickly studded with pustules, indicating fossets on the exte- 

 rior surface. These in the upper part of the valve are oval, gradually 



