CHONETES 93 



with the median portion of the valve sometimes a little flattened; car- 

 dinal area narrow, slightly concave, lying in nearly the plane of the 

 valve, delthyrium small, closed towards the apex by the convex deltidium ; 

 the cardinal margin sharply defined, bearing about three spines on 

 each side of the beak which are directed obliquely outward; the beak 

 small and but slightly extended beyond the cardinal margin, the umbo 

 inconspicuous. 



The brachial valve moderately concave in the median portion, the 

 greatest concavity near the middle, becoming flattened towards the car- 

 dinal extremities; the cardinal area narrower than that of the opposite 

 valve and meeting it in a widely obtuse angle or lying in nearly the 

 same plane ; the posterior extremity of the cardinal process tripartite 

 and filling the basal portion of the delthyrium of the opposite valve. 



Surface of both valves marked by depressed, rounded costae, about 

 four or five in the space of one millimeter, and from 35 to 40 upon 

 each valve, these costas are commonly indistinct posteriorly and 

 are frequently nearly or quite obsolete to nearly the margin of the 

 valves; upon the brachial valve a small, longitudinal, node-like prom- 

 inence is present just in front of the middle of the cardinal margin. 

 Aside from the costae the surface is marked by exceedingly fine con- 

 centric markings and by a variable number of much stronger lines of 

 growth which are sometimes nearly equally distributed, but are more 

 often crowded towards the front. 



Remarks. This little shell is similar in general form to (7. logani, 

 but it is smaller and has more nearly obsolete costag which are not 

 crenulated as are those of that species. It is associated with C. ornatus, 

 from which it may be easily distinguished by its smaller size, greater 

 relative convexity and much less sharply defined costaa. 



Horizon. Louisiana limestone. 



CHONETES BURLINGTONENSIS Weller 

 Plate VIII, Figs. 18-19 



1901. Chonetes burlingtonensis Weller, Trans. St. Louis Acad. Sci., vol. 11, 

 p. 184, pi. 16, fig. '9. 



Description. Shell semielliptical in outline, the length equal to two- 

 thirds the width, the greatest width along the hinge-line, the cardinal 

 extremities nearly rectangular, the lateral margins nearly straight or 

 slightly convex posteriorly, becoming more strongly curved anteriorly 

 and passing into the anterior margin which becomes more gently convex 

 in its median portion. The dimensions of the type specimen are : length 

 9 mm., width 13.5 mm., convexity 3 mm. 



Pedicle valve most convex posterior to the middle, the surface curving 

 rather abruptly to the beak and much more gently to the anterior margin, 



