432 NATATORES. ALCA. 
white, mixed more or less with black. In other respects 
there is little or no difference. 
Genus ALCA, Zinn. AUK. 
GENERIC CHARACTERS. 
Bit shorter than the head, nearly as deep as long, much 
compressed, cultrated; with both mandibles laterally sul- 
cated; tip of the mandible strongly arched, and hooked ; 
that of the under one obliquely truncated ; angle deep, and 
ascending. Nostrils near the lower edge, and immediately 
behind the horny part of the bill; nearly hidden by the fea- 
thers of the antie, or projecting side-angles of the forehead. 
Head narrow, and attenuated in front ; with the antise reach- 
ing as far as the nostrils. 
Wings short and narrow; and in one species unequal to 
the purpose of flight. Tail short; of twelve or sixteen fea- 
thers. 
Legs short, situated at the posterior part ef the body, and 
having the tibize hidden within the integuments of the abdo- 
men. Feet with three toes, all directed forwards, and web- 
bed ; outer toe longer than the inner, and nearly equal to the 
middle one. Claws hooked, and sharp; the middle one the 
largest and longest. Front of the tarsus and the toes scutel- 
lated. 
In this genus the bill becomes strongly compressed and 
hooked, losing that character of breadth at the base pos- 
sessed by Mergulus, and by 'TEmmincx’s genus Phaleris. 
The lateral furrows of the bill are also more distinct, in 
which respect these birds approach to the succeeding genus 
Fratercula (Puffin). Their wings are very short and nar- 
row; in one species, indeed, the Great Auk (Alca impennis), 
they are so imperfectly developed as to be totally unfit for 
