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Synopsis of the Birds 
Thalassidroma , Vigors. Hydrohates , Boie * 
Petrel-Hirondelle , Temm . 
Bill shorter than the head, slender, feeble, attenuated, ex- 
tremely compressed, hooked at the point, acute ; edges sharp, 
greatly approximated ; upper mandible slightly seamed 
each side : lower shorter, hardly angular beneath, more or 
less curved and acute at tip, end hardly distinct : nostrils 
contained in a single tube on the top of the bill : tongue 
moderate, depressed, entire, slender, acuminate, acute. Head 
small, rounded above ; eyes large ; neck short : body slender. 
Feet long, slender ; naked space of the tibia extensive; tarsus 
longer than the middle toe, slender, perfectly smooth ; toes 
rather short, slender; middle toe subequal to the outer; 
inner shortest ; the lateral margined exteriorly by a narrow 
membrane ; webs small, somewhat indented ; hind toe merely 
a slender, acute nail : nails small, rounded above, in one 
species quite flat and laminar, but generally curved, com- 
pressed, and acute. Wings long, acuminate ; first primary 
shorter than the third ;* second longest. Tail emarginate, 
or slightly forked, of twelve feathers. 
Female similar to the male. Young hardly differing from 
the adult. Moult twice a year without changing their colors. 
Colors black, with more or less of white, especially on the 
rump. Species very small : the smallest of web-footed birds. 
Serni-nocturnal : wandering. Seen all over the ocean at 
wonderful distances from land, principally at twilight, or 
ill stormy weather ; hiding from the sun in clefts of rocks, or 
in the burrows of quadrupeds. Feed on small marine ani- 
mals and seeds of sea-weeds ; very fond of greasy substan- 
* The character assigned by Mr. Vigors, of the first primary being 
longer than the fourth, though perfectly correct as to P. pelagica, cannot 
stand as generic, since in P. Leachii, it is shorter. We have not been 
able to verify the character on P. Wilsonii, the specimens we examined for 
that purpose being in moult. 
