416 PROF. HUXLEY ON THE CLASSIFICATION OF BIRDS. [Apr. II, 



6. The mandible is connected with the skull by the intermediation 

 of a quadrate bone (which represents the incus of Mammalia). 



7. Each ramus of the mandible is composed of a number of sepa- 

 rate ossifications, which may amount to as many as six in all. (Of 

 these the articxdare represents the malleus of Mammalia). 



8. The apparent "ankle-joint" is situated not between the tibia 

 and the astragalus as in the Mammalia, but between the proximal 

 and the distal divisions of the tarsus*. 



9. The brain is devoid of any coi-pus callosum. 



10. The heart is usually provided with two aortic arches ; if only 

 one remains, it is the right. 



11. The red blood-corpuscles are oval and nucleated. 



12. The cavities of the thorax and abdomen are never separated 

 by a complete diaphragm. 



13. The allantois, which is highly vascidar, is very large, and en- 

 velopes the embryo ; but no villi for placental connexion with the 

 parent are developed upon it. 



14. There are no mammary glands. 



I attach less weight to the first of these characters than to the 

 rest, since the simpler kinds of feathers very closely approach hair 

 in structure and development ; but the other thirteen are, for the 

 most part, of extreme importance, and define Birds and Reptiles, as 

 a whole, very sharply from Mammals. 



Closely as Birds approach Reptiles, however, and small as the 

 divergence of the ornithic type from the reptilian appears to be, in 

 view of the great divergences of Reptiles from one another, there are 

 still a number of characters common to Birds which are absent in 

 all recent Reptilia, and, so far as our knowledge goes, in extinct 

 Reptiles — though it must be carefully borne in mind that our infor- 

 mation respecting the latter is limited to an acquaintance with their 

 osteology. Thus — 



1. Birds possess epidermal appendages developed in sacs of the 

 dermis, and having the structure of feathers. 



2. ]\Iore or fewer of the anterior vertebrae have centra with cylin- 

 droidal articular surfaces^ . 



3. Although all birds possess a remarkably large sacrum, the 

 vertebrae, through the intervertebral foramina of which the roots of 

 the sacral plexus (and, consequently, of the great sciatic nerve) pass, 

 are not provided with expanded ribs abutting against the ilium ex- 

 ternally, and against the bodies of these vertebrae by their inner ends. 



In recent Reptiles, possessing well-developed hind limbs, the in- 

 tervertebral foramina through which the roots of the sciatic nerve 



* See Gegenbaur, 'Archiv fiir Anatomic' (1863), and ' Untersuchungen zur 

 vergleichenden Anatomie' (1864). 



t Archceopteryx may possibly prove an exception to this rule. Wiien certain 

 of the vertebrae of Birds (as in the Penguins, Larus fuscus, and others) have 

 centra with spheroidal articular surfaces, the anterior faces of the centra are con- 

 vex and the posterior concave, which is the rarest case among the Reptilia. The 

 procoelous form of vertebra, so common among the Reptilia, has not been observed 

 in the cervical or dorsal regions of the spine of Birds. 



