OSSEOUS SYSTEM OF AVES. 61 



direction in the Avosets ; extended in a straight line in the 

 Snij)es ; singularly widened and hollowed out in the Boatbills 

 ( Cmicroma, Balceniceps) ; widened, flattened, and dilated at the 

 extremity in the Spoonbill ; thickened, rounded, and bent down- 

 wards at an obtuse angle in the Flamingo, fig. 14. 



Among the Natatores, the Divers (^Coli/mbus), Grebes (^Podl- 

 ceps), and Cormorants ( Carbo) show a defective condition of the 

 bony orbits, and of the anterior parietes of the cranium; the 

 septum of the orbits is almost entirely wanting ; in place of the 

 posterior orbital parietes, there are two lacunae leading directly 

 into the cranial cavity, one superior, of large size, and one inferior, 

 smaller ; they are, in general, separated by a narrow osseous bar, 

 but in the Coulterneb {Fratercula arctica) this is also wanting, 

 so that all the orbital and optic nerves escape by a common open- 

 ing. In the Petrels and Albatrosses, the internal and posterior 

 walls of the orbits are more complete. In the Diomedea exulans 

 the optic foramina are separated both from each other and from 

 the neighbouring outlet. The occipital region is low, and divided 

 into a superior and an inferior facet, tlie latter being concave 

 from side to side. The plane of the occipital foramen is almost 

 vertical. The occipital or lambdoidal crista is well-marked, and 

 the temporal fossie nearly approximate in the middle line. In 

 these Sea-birds and in the Gulls, the lateral lacuna3 in the bony 

 parietes of the face are very considerable. 



A most remarkable characteristic of the cranium of both the 

 Brachypterous and Macropterous Sea-birds is the presence of the 

 two deep, elongated, semilunar glandular depressions extending 

 along the roof of the orbits. In the aquatic birds which frequent 

 the marshes and fresh waters, as the Anatidce or Lamelllrostres, 

 these glandular pits are wanting, or very feebly marked, as in 

 the Swans. They are, however, again met mth of large size, 

 though shallow, in the Curlews (^Numenius) and Avosets {Recur- 

 virostra) ; and are also found, though of smaller size, in the 

 Flamin2:o. 



The cranial cavity has but a limited range of size in the class 

 of Birds, although an extreme one in relation to the bulk of 

 the body : that of the smallest Humming-bird is proportionally 

 greater than in any other animal, while that of the great Dlnornis 

 is almost crocodilian in its contracted area : the size of the 

 cranium, small as it is in relation to the trunk and legs in the 

 giant bird, being expanded to the requisite extent for muscular 

 and other attachments by a thick pneiunatic cellular diploe be- 



