LOWER SASKATCHEWAN RIVER VALLEY. 
17 
ing plications attain their maximum strength and number only 
in the gerontic stage. 
In the dorsal valve the septa are somewhat rudimentary in 
ordinary specimens and shorter than the spondylium. In one 
unique specimen, however, the septa of the dorsal valve are longer 
than the spondylium of the opposite valve and take an excentric 
course across the shell terminating near the middle of the left 
half of the valve (Plate IV, figure 4), 
The maximum length in shells observed, is 31 inches. 
Horizon and Locality .— Basal dolomite beds of section at 
Grand rapids, Saskatchewan river, Fisher branch and Hilbre, 
Manitoba. The specimens figured are all from Grand rapids. 
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