THE NAUTILUS. 21 



A NEW SPHAESIUM FROM ILLINOIS. 



FRANK COLLINS BAKER. 



Sphczrium stamineum forbesi nov. var. 



Shell of good size, inflated, solid, subequilateral, trigonal ; um- 

 bones much elevated, rounded, somewhat inflated, placed a little an- 

 terior of the center of the shell, marked by very fine, concentric 

 lines of growth (sometimes coarser), the beaks very closely approxi- 

 mating ; dorsal and ventral margins well rounded ; anterior end flatly 

 rounded, posterior end plough-shaped ; both ends have a somewhat 

 truncated appearance ; umbonal slopes convexly rounded ; surface 

 inclined to be shining, lines of growth rather crowded, fine in typical 

 specimens, coarser in others; color light greenish or yellowish-horn, 

 lighter on the umbones, indistinctly rayed in some specimens ; liga- 

 ment weak, short, brownish in color ; cardinal teeth similar in form 

 and position to those of stamineum, the hinge-line not quite so thick 

 as in stamineum; lateral teeth not quite so solid as in stamineum, the 

 posterior laterals also being shorter, not reaching so high up into the 

 arch of the hinge-plait, the comparative distance between the an- 

 terior and posterior laterals being greater in forbesi than in stamineum ; 

 muscle scars and pallial line rather distinct; nacre faint bluish-white, 

 with occasional darker zones. 



Length 14.50; height 11.50; breadth 8.00 mill, types. 

 « 12.00 " 10.00 " 7.00 « " 



" 12.00 " 9.50 " 6.75 " Havana. 



" 11.00 " 8.50 " 6.50 •« 

 " 11.50 " 8.00 " 6.50 " 



Thompson's Lake, Fulton Co. (types) ; Matanzas Bay, Havana, 

 Mason Co. ; Little Fox River, White Co. Types : — Illinois State 

 Laboratory of Natural History; topotypes, Chicago Academy of 

 Sciences, Academy of Natural Sciences of Philadelphia. 



This apparently distinct variety of stamineum may be known by 

 its peculiar trigonal shape, plough-shaped posterior end and elevated, 

 inflated umbones. The ventral and dorsal margins are much more 

 rounded than in stamineum and the lateral teeth are farther apart. 

 The umbonal sculpture is typically very fine, but is also as coarse as 

 typical stamineum in some specimens. 



The variety will probably prove to be a common form in many 

 localities and will be easily recognized and separated from typical 



