1. STERNOTH©RUS. 193 
A. Beak strongly hooked; interorbital 
width considerably less than the 
longitudinal suture between the two 
BRO THOUS —o5 Win Doe Om OwicIOo 1. niger, p. 194. 
B. Beak notched mesially, with a very 
slight, obtuse cusp on each side of 
the notch; interorbital width con- 
siderably less than the longitudinal 
suture between the frontal shields . 2. sinwatus, p. 194. 
Fig. 47. 
Shell of Sternotherus derbianus. 
C. Beak neither hooked nor bicuspid; frontal suture not or 
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 
humeral, and does not exceed the length 
of the inner border of the latter shield.. 4. derbranus, p. 195. 
