PISIDIUM. 59 



43. Sphaerilim formosum, MEEK and HAYDEN. &quot; Shell small, 

 oval, oblique, scarcely ventricose ; cardinal margin straight ; buccal end 

 rounded ; anal extremity obliquely truncate ; basal margin semielliptical 

 or broadly rounded ; beaks obtuse, tumid, rising somewhat above the 

 hinge, nearly touching, placed a little in advance of the middle ; surface 

 ornamented by very fine, regular, distinct, concentric wrinkles.&quot; 



Long. 0.17; Lat. 0.08; Diam. 0.14 inches. 



Hab. Three miles above Fort Union, Nebraska, United States of America. 

 Tertiary formation. (Cabinet of Smithsonian Institution.) 



Cyclas formosa, MEEK and HAYDEN, Proc. Acad. Nat. Sci. Phila. 



1856, 115. 



Cyclas fragilis^ MEEK and HAYDEN, loc. sub. cit. 1856, 115. 

 Sph&rium formosum, MEEK and HAYDEN, ib. May, 1860, 185. 



44. Sphaerium subellipticum, MEEK and HAYDEN. &quot; Shell 

 small, elliptical-ovate, somewhat ventricose, thin and fragile ; posterior 

 end narrower than the anterior, both narrowly rounded ; base semi- 

 elliptical oij semiovate ; cardinal border apparently rounding gradually to 

 both extremities ; beaks not much elevated, pointed, incurved, not oblique, 

 located near the middle ; surface indistinctly marked with lines of growth.&quot; 



Long. 0.24; Lat. 0.14 inches. 



Hab. Three miles above Fort Union, Nebraska, United States of America. 

 Tertiary formation. (Cabinet of Smithsonian Institution.) 



Cyclas subelliptica, MEEK and HAYDEN, Proc. Acad. Nat. Sci. Phila. 



1856, 115. 

 Sphxrium subellipticum, MEEK and HAYDEN, ib. May, 1860, 185. 



&quot; The beaks are so near the middle, and curved so nearly at 

 right angles to the longitudinal diameter of the shell, that it is 

 not easy to determine, especially from the examination of muti 

 lated specimens, which is the posterior or which the anterior end. 

 As we have only seen imperfect specimens, we are not sure the 

 surface markings are indistinct on unworn shells.&quot; M. & II. 



PISIDIUM, PFEIFFER. 



Pectunculus, LIST. 1685. Musculus, GAULT. 1742. Tellina, MULLER, 

 m4. Sphaerium, SCOP. 1777. Cardt um, POLI, 1791. Cyclas, 

 LAMK. 1818. Pera, EnyJesia,Cordu/a, LEACH, 1820. Physemoda, 

 RAF. 1820. Pisidium, PF. l&2I. Gallileja, DA COSTA, 1839. 

 Pisum, GRAY (non MEOERLE), 1847. Musculium, GRAY (non 

 LINK), 1851. 



Animal elongate-oval, compressed laterally ; lobes of the 

 mantle without tentacles, united posteriorly into a single, short 



1 Pfeiffer, Deutsch. moll. 1821. 



