PRODUCTELLA 97 



on each side of the beak. The cardinal area, delthyrium, and internal 

 characters of the valve not observed. 



Brachial valve moderately concave with the greatest concavity near the 

 middle, the surface becoming flattened towards the cardinal extremities ; 

 at the beak or initial point of the valve, close to the cardinal margin, is a 

 small, round, node-like protuberance. The cardinal area, cardinal process, 

 and internal features of the shell not observed. 



Surface of both valves marked by fine, thread-like, wavy, discontinu- 

 ous, more or less irregular, radiating costae, about six of which occupy the 

 space of one millimeter, these are crossed by still finer concentric striae 

 which give to the uninjured shell surface a finely reticulated ornamenta- 

 tion when examined under a lens. The older half of the shell is also fre- 

 quently marked by two sets of more or less conspicuous concentric 

 wrinkles which cross each other in the manner of the engine-turned mark- 

 ings on a watch case. Irregular concentric lines of growth in the form of 

 more or less prominent wrinkles are not infrequently present. 



Remarks. This species is the only known member of the genus, and is 

 especially characteristic of the Kinderhook beds 2, 3 and 4 of the Burling- 

 ton, Iowa, section. The specimens sometimes occur in enormous numbers 

 in the lower of these beds, a yellow sandstone, but it always occurs in this 

 bed in the condition of modified casts and moulds which do not exhibit 

 the true characters of the surface markings to best advantage. These 

 markings are better shown on specimens from the limestone beds No. 3 

 and 4, except the two sets of crossed concentric marks which usually are 

 more conspicuous upon the sandstone specimens, and which seem to be 

 more conspicuous upon the brachial than upon the pedicle valves. 



The species is exceedingly variable in the proportional length and 

 breadth of the shell, some specimens being considerably longer than wide, 

 while others are conspicuously transverse. The specimens whose dimen- 

 sions have been given are of about average size and form. 



Horizon. Kinderhook. 



Genus PRODUCTELLA Hall 



Descrip tion. Shells below medium size, more or less deeply concavo- 

 convex, the hinge-line straight, equaling or shorter than the 

 greatest width of the shell. Pedicle valve convex, the beak more or less 

 incurved, dependent upon the amount of convexity of the valve ; a narrow 

 cardinal area present, with the delthyrium closed by a deltidium ; cardinal 

 teeth present. Brachial valve concave, furnished with dental sockets and 

 socket plates. External surface of the plates spinose, the spines usually 

 arranged more or less irregularly, also marked by irregular, concentric or 

 radiating marks, or by both. 



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