480 MISSISSIPPIAN BRACHIOPODA 



are divided into flattened spines, about three of which occupy 1 mm., 

 the spines of successive rows are usually somewhat regularly arranged 

 in radial series so that the surface of the shell, when not exfoliated, has 

 the appearance of being finely costate. 



Remarks. This species exhibits considerable variation in size, although in 

 its more typical form it rarely exceeds 10 mmu in length. The propor- 

 tional dimensions of the shell are also somewhat variable, the width 

 being sometimes greater and sometimes less than the length, though 

 the width is more commonly equal to or a little greater than the length. 

 The species is most common and has its most typical development in the 

 fauna of the Salem limestone, but it occurs also in beds both higher 

 and lower than that horizon. The species most closely resembles C. 

 parvirostra, but it is smaller and usually more nearly subcircular in outline, 

 although C. parvirostra has sometimes been interpreted as a large form of 

 C. hirsuta. The species also resembles C. sublamellosa, but besides being a 

 smaller species its valves are subequally convex, while the brachial valve of 

 C. sublamellosa is notably more convex than the pedicle. It is not improba- 

 ble that the young individuals of C. sublamellosa have more nearly equally 

 convex valves than the mature shells, and that those individuals in the 

 Chester which have sometimes been identified as C. hirsuta are in reality 

 the young of S. sublamellosa. 



Horizon. Warsaw formation, Salem limestone, Ste. Genevieve limestone. 



CLIOTHYRIDINA LENTICULARIS n. sp. 

 Plate LXXX, Figs. 25-30 



Description. Shell small, lenticular in form, subelliptical to subpen- 

 tagonal in outline, the width greater than the length, the hinge-line 

 much shorter than the greatest width of the shell. The dimensions of 

 two individuals are : length 10 mm. and 11 mm., width 10.5 mm. and 

 13.3 mm., thickness 5.8 mm. and 6 mm. 



Pedicle valve moderately convex, the greatest convexity posterior to 

 the middle, the surface slightly compressed towards the cardinal ex- 

 tremities, curving rather abruptly from the point of greatest convexity to 

 the cardinal margin and more gently to the lateral margins, in the 

 median portion the surface of the valve curves with increasing convexity 

 to the anterior margin; mesial sinus obsolete except towards the front 

 of the valve, where a broad, flattened area is developed which is depressed 

 slightly or not at all below the general surface, anteriorly this flattened 

 area curves somewhat abruptly towards the opposite valve and is pro- 

 duced beyond the general outline of the valve ; the beak small, a little 

 incurved, pierced by a subcircular foramen; cardinal area obsolete, the 

 delthyrium connecting with the foramen at its apex, filled by the beak 

 of the opposite valve. 



