CHONOPECTUS 95 



the front and rounding regularly into the anterior margin which is some- 

 what straightened in its median portion. The dimensions of two speci- 

 mens are: length 8.9 mm. and 8.7 mm., width 11.9 mm. and 11.1 mm., con- 

 vexity 2.8 mm. and 2.8 mm. 



Pedicle valve most convex near or a little in front of the middle, the 

 surface commonly curving more abruptly to the anterior margin than to 

 the beak, slightly compressed towards the cardinal extremities but only 

 obscurely auriculate; mesial sinus obsolete, although the mesial portion 

 of the valve is sometimes slightly flattened; umbonal region somewhat 

 flattened, the beak not produced beyond the cardinal margin ; the cardinal 

 margin sharply defined, bearing four or five oblique spines on each side 

 of the beak ; cardinal area rather broad and flat, the delthyrium broadly 

 triangular with a small deltidium closing its apical portion, the lateral 

 margins sharply defined, bearing four or five, probably oblique, spines on 

 each side of the beak. 



Brachial valve only moderately concave, the greatest concavity in front 

 of the middle, becoming a little flattened towards the cardinal extremi- 

 ties ; the cardinal area very narrow. Internally the cardinal process is of 

 moderate size and is flanked by a pair of widely divergent, rather short 

 ridges, in the outer sides of which the dental sockets are excavated; the 

 muscular scars rather indistinct; the surface covered with elongate pa- 

 pillae which are arranged in radial series and grow more prominent 

 towards the outer margins of the valve. 



Surface of both valves appearing nearly smooth to the naked eye, but a 

 lens shows that obscure and more or less discontinuous radiating costa? 

 are present, and that these are crossed by minute, raised, flexuous and 

 discontinuous concentric markings. 



Remarks. This species has only been observed from the typical locality 

 in Monroe County, Illinois, and in Ste. Genevieve County, Missouri, where 

 it sometimes occurs in abundance in certain of the limestone layers. The 

 species is easily recognized because of the nearly obsolete surface mark- 

 ings. The raised concentric markings of the shell somewhat resemble 

 those of C. logani, but they do not have the regularity of the similar mark- 

 ings of that species, and are more nearly continuous because of the nearly 

 obsolete radiating costas. Other characters which distinguish the species 

 are the slight auriculation of the shell and the anterior position of the 

 greatest convexity of the pedicle valve. 



Horizon. Upper Keokuk limestone. 



Genus CHONOPECTUS Hall and Clarke 



Description. Shell concavo-convex, broader than long, the greatest width 

 at or near the hinge-line ; cardinal area of the pedicle valve narrow, its 

 lateral margins bearing a single row of slightly divergent spines which 



