18® 
PALAEONTOLOGY OF NEW-YORK. 
Prodiictella costatula (n. s.). 
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 costse, from which, at intervals, rise 
slender spines. In the young shells, and the upper half of the older 
ones, the costse 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 costse 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 surface 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 
