160 Trans. Acad. Set. of St. Louis. 



nent, beak scarcely elevated above the hinge margin. Pedicle 

 valve nearly plain, slightly convex towards the beak, flattened 

 at the lateral margins, and slightly concave towards the basal 

 margin which is not sinuate ; area long, narrow ; delthyrium 

 broad." 



" Surface marked by alternating larger and smaller striae, 

 which in the casts appear to be fasciculate near the margins, 

 striae curved upwards on the margin of the convex valve, but 

 not running out on the hinge line." 



The dimensions of an average specimen are, length 19 mm., 

 width 22 mm., convexity of brachial valve 6 mm. 



Remarks. This species is closely allied to 0. chemungensis 

 of the New York Devonian faunas. Its form is more regular 

 than in most of the species of the genus, but it does exhibit 

 some variation, especially in the length of the hinge-line 

 which is often shorter than the width of the shell. The 

 pedicle valve is more variable than the brachial and has a 

 greater resemblance to 0. chemungensis. 



Productus arcuatus Hall. 



PI. XIV. f. 23. 



This species is rarely represented in this fauna, its normal 

 position being in the oolitic limestone above. The specimens 

 which have been observed are all imperfect, but they retain 

 the general form, proportion and markings of the species 

 and are probably identical with those in the oolite bed. 



Productus parvulus Win. 



PI. XIV f. 21-22. 



Original description. " Shell very small, semi-elliptic or 

 nearly semicircular in outline, with a hinge-line equalling the 

 greatest width, or a little less. Pedicle valve ventricose, with 

 regular, though slightly diminishing curvature from beak to 

 anterior margin, describing an arc of about 180°; beak ele- 

 vated above the hinge- line and incurved over it; flanks regu- 

 larly convex, abruptly flattened and auriculate at the hinge 

 extremities. Brachial valve unknown. Surface ornamented 

 with small, rigid, continuous, radiating ribs, which on the 

 sides increase by implantation." 



