FOSSILS OF THE ST. LOUIS GROUP. 539 



lias only four pseudo-auibulacral areas. As this agrees with the, others, 

 however, exactly, iu all its specific characters, it is evidently a inon- 

 -ity of the same species, produced by the non-development of one 

 of the radial pieces, by which means two of the pseudo-ambulacral 

 fields are, as it were, welded together, to form one larger than any of 

 the other three. 



Locality and position St. Louis division of the Lower Carboniferous 

 series, iu Hardiu county, Illinois. 



AIOLLUSCA, 



LAMELLIBRANCHIATA. 



LlTHOPHAGA? PERTEXUIS, M. and W. 



PI. 22, Fig. 1. 

 Lithophaga? pertenuig, MEEK and WOBTHEX, 1865. Proceed. A.cad. Jfat. Sci., Phila., p. 245. 



SHELL slender, elongated, narrowing anteriorly, ex- 

 tremely thin, moderately convex in the central and ante- 

 rior regions, more compressed and cnneate beliind; pos- 

 terior margin narrowly ronnded in outline ; anterior margin 

 extremely short and very narrowly ronnded; basal margin 

 straight along the middle and curved up gradually towards 

 the extremities; hinge line straight, not exactly parallel to 

 the base, and apparently about half as long as the shell, 

 passing imperceptibly into the posterior dorsal margin. 

 Beaks almost terminal, very oblique, and nearly obsolete. 

 Surface smooth, or with only faint traces of fine, concen- 

 tric stria?, and very obscure, undefined, concentric undula- 

 tions. 



Length, 1.73 inch; hight, 0.62 inch; convexity, 0.50 inch. 



This species has nearly the form and general appearance of that 

 which we have with doubt referred to Lithophaga ? (Modiola) lingualis, 

 of Phillips, from the Keokuk Limestone, but may be distinguished by 

 its smooth surface, which never shows the distinct thread-like concen- 

 tric striae and regular wrinkles of that shell. As we know nothing of 

 the hinge and interior of these species, we merely place them provision- 



