80 Trans. Acad. Sci. of St. Louis. 



Louisiana, Missouri,* varying greatly in other characters, 

 such as length, breadth, thickness, curvature of the area and 

 length of hinge-line, this angle remains approximately the 

 same, being not far from 60°. If the size of this angle may 

 be considered as a specific character of value, then S. exten- 

 uaius is certainly a good species while S. halli may be only a 

 small variety of S. hannibalensis. Schuchert and others 

 have considered S. hannibalensis as identical with 8. Carteri 

 Hall, and it is quite possible that this is true, but it is 

 certainly a mistake to consider S. halli as a synonym of S. 

 extenuatus. 



Reticularia cooperensis (Swall.). 



Pi. ii. f. 11. 



Shell transversely subelliptical in outline, the hinge-line very 

 short, the cardinal angles rounded. Pedicle valve rather 

 strongly convex, the greatest convexity between the middle 

 of the valve and the beak; the beak moderately acute, in- 

 curved; the cardinal area small, a large portion of it occupied 

 by the delthyrium, its margins rounded ; mesial sinus shallow, 

 undefined, sometimes almost obsolete. Brachial valve less 

 convex than the opposite one, its greatest convexity at 

 or a little posterior to the middle; the mesial fold ill- 

 defined, not elevated above the general surface of the valve. 

 Surface marked by more or less conspicuous concentric lines 

 of growth, and also by very fine radiating costae which form 

 little pustules at the margins of the concentric ridges, indi- 

 cating the presence of concentric rows of fine spines upon the 

 surface of the shell. Dimensions of a rather large specimen: 

 length of brachial valve, 18 mm., breadth, 25 mm. 



Remarks. The types of R. hirla (W. and W.) are from 

 the <k Yellow sandstone " at Burlington, but apparently from 

 the upper formation included under this name. In the Chono- 

 pectus sandstone the species is not common, the brachial 

 valve illustrated being the only one certainly identified from 

 that formation. R. hirla is generally considered as synony- 

 mous with R. cooperensis (Swall.) from the Chouteau lime- 



* These specimens were generously loaned for study by Mr. R. R. Rowley 

 of Louisiana. 



