FOSSILS OF THE ST. LOUIS GKOTJP. 541 



dorsal margin less than tke entire lengtk of the skell, and 

 inflected so as to form a narrow but well defined escutck- 

 eon. or false area. Beaks narrow, or compressed antero- 

 posteriorly, rather pointed, prominent and incurved, near- 

 ly terminal, or placed directly over the anterior margin. 

 Uniboiial slopes oblique, very prominent near the beaks, 

 but less so along tke central and posterior ventral region; 

 anterior and ventral regions abruptly cuneate, with a very 

 faint, undefined impression extending from the beaks ob- 

 liquely backward toward tke middle of tke base, just in 

 front of tke umbonal prominence. Surface ornamented 

 witk small, regular, concentric undulations, with appar- 

 ently very faint indications of very small, radiating striae. 



Lengtk, 1.17 inckes; kigkt from ventral to cardinal mar- 

 gin, 0.80 hick; do. to summit of beaks, 0.90 inck; con- 

 vexity, 0.65 inck; gap of valves bekiud, 0.25 inck. 



Although this species seems to agree in most of its known characters 

 with the types upon which the genus Clmnomya was established, it 

 differs in being a proportionally shorter, less widely gaping shell, while 

 its beaks are more prominent and oblique. As we know nothing of its 

 hinge or interior, or of its finer surface markings, it was only provision- 

 ally that we at first placed it in the genus Chccnomya, and that we now 

 leave it there. Possibly, we should call it AUorisima rhomboidalis, or 

 Sedgirick ia rJtoni bo idali*. 



Locality and position St. Louis Limestone, of the Lower Carbonifer- 

 ous series ; near Alton, Illinois. 



PTEROPODA. 



GEXUS COXULARIA. 



COXULAKIA MISSOURIEXSIS, Swallow ? 



PL 22, Fig. 5. 

 'laria Xitsaurienrit, SWALLOW, 1860. Trans. St. Louis Acad. Sci., VoL I, p. 637. 



Sm.u. attaining a large size, presenting tke usual 

 elongated, four-sided, pyramidal form, two of tke opposite 

 sides being wider tkaii tke otkers, witk tkeir lateral mar- 



