: 
39 
living many. years in the Society’s Gardens. That of P. riippellit 
was received from Eastern Africa in June 1858. 
Comparing, first of all, the skulls of these two birds together, we 
see that the frontal protuberance, which in P. gambensis (fig. 1) is 
hardly elevated 0-2 inch above the general level, rises to an enormous 
size in P. réippellii (fig. 2), attaining a height of 1°05, a breadth of 
0°75, and a length from back to front of 1-65. It may also be re- 
marked, that, from the hard. character of the osseous structure in 
the protuberance of P. gambensis, it is obvious that it has reached 
its maximum of development. The outlines of the two skulls are 
represented in the accompanying woodcuts. 
Fig. 2. 
Their conformation is otherwise generally similar, that of P. riip- 
pellii being slightly narrower, and rather longer. It may be re- 
marked, however, that the skull of P. riippellit is broader between 
the orbits; but that, drawing a vertical line from the middle of the 
space between the nostrils to a base-line joining the edges of the upper 
mandibles, and comparing them at this point, it is here narrower and 
more elevated; the proportion of the vertical to the base being in P. 
riippellii about 3:5, in P. gambensis about 7:9. The depressed 
space between the protuberance and the naked part of the bill is also 
somewhat differently shaped in the two birds. In P. riippellit the 
outline of this space next to the protuberance forms a segment of a 
circle of which the centre is at the junction-point of the two other 
