184 Trans. Acad. Sci. of St. Louis. 



ones and the hinge-line is relatively shorter. Usually two 

 spine bases can be detected on the cardinal margin each side 

 of the beak, and sometimes a third one. The spines them- 

 selves are oblique in position. On the larger shells the num- 

 ber of plications sometimes reaches forty. 



Chonetes burlingtonensis n. sp. 



Pi. xvi. f. 9. 



Shell of medium size, semielliptical in outline, the hinge- 

 line as long as or a little shorter than the greatest width of 

 the shell. Pedicle valve prominent on the umbo, compressed 

 towards the cardinal angles, and flattened or slightly sinuate 

 along the median line. The bases of two oblique spines on 

 the cardinal margin may usually be seen on each side of the 

 beak. Surface of the pedicle valve ornamented with about 

 100 rounded plications on the margins of the shell, which 

 originate by bifurcation from less than 25 at the beak, and 

 which are furnished with numerous tubular openings. The 

 furrows between the plications are narrower than the plica- 

 tions themselves. Besides the plications the shell is marked 

 by exceedingly fine, inconspicuous concentric striae, which are 

 strongest in the radiating furrows. Brachial valve unknown. 



The dimensions of an average sized specimen are : length 9 

 mm., breadth 14 mm., and convexity 3 mm. 



Remarks. This species is less common in the oolite fauna 

 than C. logani, from which it can be easily distinguished by 

 its larger size, its greater number of plications, its less con- 

 vexity, its more compressed cardinal angles, and by the ab- 

 sence of the conspicuous concentric striae. In size and gen- 

 eral outline the species resembles C. illinoisensis, but it differs 

 from this common species of the Burlington limestone in its 

 smaller number of plications and in its smaller number of 

 cardinal spines. 



Productella concentrica (Hall). 



PL XVI. f. 12-14. 



Original description.* "Shell small, semi-elliptical; 

 hinge-line scarcely so long as the greatest width of the shell. 



* This is the description published by Hall in 1858 in Rep. Geol. Surv. la. 

 I 2 : 517. An earlier description was published in 1857 by the same author 

 in the Reg. Rep. of N. Y. State Mus. Nat. Hist. 



