244 C. E. Beecher — IT. A. Species of Strophalosia. 



little shorter than the width of the shell, with a narrow trian- 

 gular central fissure, covered by a pedicle-sheath ; teeth well- 

 developed; cardinal muscular scars small, obscure, confined to 

 the umbonal cavity. 



A specimen of medium size measures 2 - 6 mm in width, and 

 l-6 mm in length. 



Some examples, as in figure 11, are without marginal spines, 

 and none have shown them developed along the hinge, as is 

 characteristic of the other species here described. All the 

 individuals observed have been found attached to larger forms 

 of brachiopods of the genera Productella, Spirifer, Syringo- 

 thyris, and Cyrtina. 



Geological position. — From the Choteau limestone, Pike 

 County, Missouri. Collected by R. R. Rowley, and loaned to 

 the writer by Charles Schuchert. 



Strophalosia Keokuk sp. nov. (Plate IX, figures 18-24). 



Shell broadly elliptical or ovate, truncate along the hinge, 

 greatest width usually near the middle of the length. Ventral 

 valve attached by its central portion ; margins furnished with 

 numerous, often crowded, spines serving as additional points 

 of attachment to foreign objects ; area triangular, with a nar- 

 row fissure covered by the pedicle-sheath ; cardinal teeth well- 

 developed ; muscular scars faintly defined and separated by an 

 obscure median septum ; interior finely pustulose and marked 

 about the margins by openings leading into the tubular spines. 

 Dorsal valve concave, convex on the umbo and beak ; surface 

 smooth or marked by a few concentric wrinkles ; cardinal pro- 

 cess present ; interior not observed. 



The largest specimen observed has a width of ll mm , and an 

 individual of average size measures 6"5 mm in width and 4 , 8 mm in 

 length. 



All the specimens which have thus far been found are on 

 shells of Platyceras. Many of the latter were attached to the 

 tegmina of crinoids, as they frequently occur at Crawfordsville, 

 Indiana. In a collection comprising about two hundred Platy- 

 ceras, twenty were found supporting Strophalosia, and sixty- 

 seven valves of this genus were observed on these specimens. 



Mr. Charles Schuchert has called the writer's attention to a 

 specimen of Productus horndus Sow., on which are one 

 mature and eight young individuals of Strophalosia Goldftissi 

 Miinster. The larval shells bear a very close resemblance to 

 the specimens of S. Keokuk here described, and in their early 

 stages of growth both species are indistinguishable from each 

 other. S. Goldfussi, however, develops spines on the dorsal 

 valve at an early period, while none have yet been seen on 

 specimens of S. Keokuk. 



