CRIMSON NUT-CRACKER. 
159 
gradual deyelopement and diminution of the strength 
and form of the bill in this circular group will he 
made intelligible by the following outlines of the 
hills of the suh-genera which compose the circle. 
2 1 
S 4 3 
It is obvious that those hills are the strongest 
whose mandibles are of equal thickness and of un- 
usual shortness ; we consequently find that it is the 
pre-eminent distinction of the two typical genera, 
Pyrenestes, fig. 1, and Coccoborus, fig. 2 ; all the others 
having the bill either more lengthened, or the under 
mandible much weaker than the upper. But this 
change is nevertheless gradual. Pyrenestes, fig. 1, 
for instance, has the most conic bill of all, for it has 
no notch at the tip. Then comes Dertroides, fig. 5, 
putting on the appearance of a hombill or a buceros, 
not only on the knobbed front, but in the situation of 
the nostrils ; its bill is something like the last, hut it 
is less conic and more lengthened. From this form 
we pass to Spermophaga, fig. 4, where the hill has 
