1894.] NATURAL SCIENCES OF PHILADELPHIA. 77 



2-sIit, the posterior portiou strongly directed forward . 



Metiiroplux 

 bb. Posterior insertiou- plate of valve viii spreading backward 

 or vertical; nuicro not at the posterior extremity, 

 c. Valve viii with two slits, and a wide, shallow posterior 



sinus Aeanthochltes 



cc. Valve viii with several slits Notoplax 



Key to Australian species of Acanthochites. 



a. Anterior valve having five strong radiating ribs; insertion- 

 plate of tail valve with more than two slits .... eostaUis 

 an. Anterior valve not strongly ribbed. 



b. Posterior insertion- plate of tail valve directed forward, 

 two-slit. 

 c. Posterior margins of median valves convex (or straight 

 by erosion); side areas of valves coarsely granulose, 

 the dorsal areas smooth, not defined, not longitudi- 

 nally striated. Size small retro jecttii< 



bb. Posterior insertion-plate directed backward or vertical; 

 mucro not at the posterior extremity, 

 c. Valve viii with two slits, a wide, shallow sinus 

 between them. 

 d. Tegmentum of valve viii less than half as wide 

 as those of the intermediate valves; dorsal 

 areas smoother than sides, but not defined, 

 not longitudinally striated; posterior margins 

 of valves very convex, broadly reflexed 

 within; tufts dense, silvery, asbestus-like . . 



asbestoicles 

 del. Tegmentum of valve viii more than half as 

 wide as those of intermediate valves; dorsal 

 areas longitudinally striated ; posterior mar- 

 gins of valves i to vii not convex, generally 

 concave. 

 e. Posterior margin of the insertion-plate of 

 valve viii regularly convex, not bilobed ; 

 sides of valves with radially elongated 

 convex pustules; interior and sutural- 

 laminte roseate; tufts inconspicuous, hardly 



