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tively coarse teeth, the first three (from the imho) angular, the 

 remainder straight and almost horizontal; musaie scars relatively 



Isr-e, weaklT but^:ressed; inner margin of the valve ;7ith fine 



crenulations. 



Dimensions .- Len-th. 26 nim. ; alt.. 25.8 mn. ; semidiam., 9 nm,. 



This interesting ai-cymcris is represented by three right 

 valves. The smallest form has radial soulpt^j-e which differs some- 

 what from that of the largest, the Hbs being very sharply defined 

 and subangular, beoomiiig slightly broader distally, with faint sec- 

 ondaries in the distal portion of some of the interspaces. On the 

 shell of intenr.ediate size the primary ribs are relatively broad 

 and angular distally, with secondaries of almost the same order 

 of magnitude. Finally, on the largest individual, the ribs are 

 flat, broad and grooved, -rom the above It is quite obvious that 

 -oung shells display to better advantage the characteristic can- 

 cellation. 



The deeply immerse.'., .nlmo;-^t i-ternal ligament, pronounced 

 backv/ard twisting of the umbo, the strikingly different anterior 

 and Toosterior teeth are unusual for the genus. The largest Bow- 

 den valve bears a remarkably close resemblance to a lar-er Jest 

 Indian v Ive in the national I.aiseum collection. It should be 

 remembered that lamarck (1855) stated that this spe-^ies is "re- 

 marquable surtout par les crochets qui or.t leur pointe dirigde 



