SCHELLWIENELLA 63 



SCHELLWIENELLA CBENULICOSTATA n. sp. 



Plate III, Figs. 17-18 



Description. Shell concavo-convex, a little above medium size, broader 

 than long, the greatest width near or posterior to the mid-length, the 

 hinge-line a little shorter than the greatest width, the cardinal extremities 

 angular. The dimensions of a very perfect pedicle valve are: length 25.8 

 mm., greatest width 35 mm., length of hinge-line 30 mm., height of car- 

 dinal area 2.6 mm. 



Pedicle valve concave, the greatest concavity near the middle, the 

 surface flattened towards the cardinal extremities, a small umbonal region 

 rather abruptly elevated from the general surface ; mesial sinus obsolete ; 

 the beak small, nearly 'erect; cardinal area narrow, lying at nearly a right 

 angle to the plane of the valve, the lateral margins sharply defined, nearly 

 straight or gently convex from the cardinal extremities nearly to the beak, 

 where they become concave and curve up rather abruptly to the apex 

 of the valve, the lateral margins on the two sides of the beak lying in 

 essentially a straight line; delthyrium broadly triangular, twice or more 

 than twice as wide as high with a convex deltidium closing the apical 

 portion. Internally a pair of widely divergent hinge-teeth of moderate 

 size are present ; the other internal characters not observed but the dental 

 plates are probably restricted to ridge-like thickenings of the inner sur- 

 face of the shell at each side of the delthyrium. 



Brachial valve not observed. 



Surface of the shell marked by fine, uniform, regularly dividing, rounded 

 costas, about four or sometimes five of which occupy the space of 1 mm., 

 \vith the intercostal furrows narrower than the costae. Crossing the 

 costae there are fine, concentric markings, becoming more conspicuous 

 beyond the middle of the valve where they divide the summits of the 

 radiating costa? into a series of crenulations. Besides the fine concentric 

 markings there are a few much stronger, concentric lines of growth 

 which are more conspicuous towards the outer margin. 



Remarks. This species has been described almost exclusively from a 

 very perfect pedicle valve from the Kinderhook oolite at Burlington, 

 Iowa, but it is recognized also, from the pedicle valve alone, in the sub- 

 jacent yellow sandstone at the same locality. In general outline the shell 

 is not greatly different from 8. inflatus or 8. inaqualis. It differs from 

 both these species in the size and form of the radiating costae. The pedicle 

 valve of the species may be distinguished from that of 8. inflatus by its 

 much narrower cardinal area, by the nearly straight line formed by the 

 two lateral margins of the area, and by the small, abruptly elevated 

 umbonal region of the valve. From 8. incequalis the species differs, in 

 addition to the different character of the costae, in the greater concavity 



