1. STERNOTn.lJETTS. 



193 



A. Beak stroiip:]y hooked ; iiifororhifal 



width considerably less than (ho 

 longitudinal suturo between the two 

 frontal shields 1. niger, p. 194. 



B. Beak notched mesially, with a very 



sli<i;ht, obtuse cusp on each side of 

 the notch ; interorbital Avidth con- 

 siderably less than the lonp:itudinal 

 suture between the frontal shields . 2. sinnatus, p. 1 94, 



Fig. 47. 



Shell of Sternotharus derhianus. 



C. Beak neither hooked nor bicuspid; frontal suture not 

 but slightly exceeding the width of the interorbital space. 



The length of the outer border of the pec- 

 toral shield equals that of the humeral . 3. nigricans, p. 195. 



The length of the outer border of the pec- 

 toral is much less than that of the 

 hiimeral, and does not exceed the length 

 of the inner border of the latter shield. . 4. derhianus, p. 195. 



or 



