98 TRANS. OF THE ACAD. OF SCIENCE. 



AVICULA MAGNA. 



Shell very large, ovate; hinge line oblique, nearly as long 

 as the shell. Left valve somewhat regularly convex, flattened 

 towards the posterior margin ; umbo full ; beak projecting be- 

 yond the cardinal line: anterior ear well defined, triangular; 

 separated from the lateral margin by a deep sinus ; anterior 

 extremity acute : posterior ear not defined by a sinus on the 

 margin ; outer angle obtuse : surface marked by large radia- 

 ting plications and distinct lines of growth. Right valve un- 

 known. 



Length, 3.25 ; height, 3.55. Length of anterior wing, 1.26 ; 

 of posterior wing, 1.27. 



This shell was discovered in the Archimedes Limestone of 

 Knox county, Missouri. 



CONULARIA OSAGENSIS. 



Shell large, pyramidal ; transverse section, forming a paral- 

 lelogram with the shorter sides, convex, and about half as 

 long as the other sides, which are but slightly convex; each 

 corner has a deep angular sulcation ; the sides are marked 

 with high, narrow, transverse costre (the spaces concave and 

 much wider than the costce), with a line or ridge along the 

 middle of the wider sides; the ribs on each side of the mesial 

 line are* arched, convex towards the base of the shell, and do 

 not always meet at the mesial line ; the costse on the shorter 

 sides are more arched, and are also convex towards the base 

 of the shell. The surface is polished. 



This fossil is rare in the upper bed of the Archimedes 

 Limestone near Boonville. Collected by Dr. Giddings. 



Orthoceras Chesterense. 



Shell tapers rapidly; transverse section orbicular; the cast 

 is marked with subearinated annulations — about seven in a 

 space equal to the diameter — the space between regularly 

 concave. 



This fossil is rare in the Ste. Genevieve Limestone at Ches- 

 ter, Illinois, and near St. Mary's Missouri. 



EtTOMPHALUS PERSPECTIVUS. 



Shell small, depressed, subcorneal : volutions five to seven ; 

 sharply carinated on the upper exterior margin and on the 

 middle of the lower surface; flat or slightly concave on the 

 upper side; slightly convex and nearly perpendicular on the 

 outer surface of the inner volutions, causing the spire to as- 

 cend by regular steps : umbilicus broad, deep, infundibuli- 



