26 ME. W. P. PTCRAFT ON THE 



The component elements of the typical vertebral column may be expressed as 



follows : — 



S.Sc. 



A 



Cv. 11 : Cv.Th. 3. Th. 5 + 2 : Lb. 4. Lb.Sc. 3 : Sc. 2 : Cd. 4 + 8 =42. 

 7 12 



It should be stated that the evidence for the excalation of vertebrae, which, it is 

 contended has taken place, rests upon the fact that the presacral vertebrae show no trace 

 whatever of fusion of elements, and the nerve-apertures are all perfectly normal. This is 

 a point of some importance, since in the vertebral columns of Amphibia possessing fewer 

 than the normal number of vertebrae, the reduction appears generally to have been 

 brought about by the fusion or confluence of vertebrae, rather than by excalation. 

 The reduction in the number of the caudal vertebrae among the Aves, however, is, as we 

 have just indicated, undoubtedly brought about by the absorption of the vertebrae lying 

 immediately in front of the pygostyle. This is well seen in Brit. Mus. 28.5.7.39, showing 

 the remains of the 7th free caudal, which consists, now, of a small portion of the centrum 

 only, and this forms a wedge between the 6th vertebra and the pygostyle. Later in life 

 this wedge fuses with, and forms part of, the pygostyle itself. 



It may be contended that the inconstancy in the number of presacral vertebrae is due 

 to what Mr. Bateson calls " Meristic variation." It seems to me, however, that the facts 

 submitted rather favour the view that a genei-al and orderly reduction in the length of 

 the vertebral column is taking place, rather than that which implies simj)ly a series of 

 sporadic variations of no apparent meaning. Before this matter can be settled satis- 

 factorily, it will be necessary to examine a much larger series of individuals of the same 

 species, and, if possible, of nestlings from the same nest. 



V. The Ribs. 



The cervical ribs are styloid and short. The heads of the ribs have the form of flattened 

 band-like lamellae and fuse above with the diapophysis, below with the catapophysis of 

 each vertebra, thus forming a canal for the vertebral artery. The head and the short 

 shaft are sharply contrasted one with another ; so that, in a lateral view, the pleurapo- 

 physeal lamella stands out buttress-fashion against the centrum. The hindmost cervicals 

 or cervico-thoracics, however, must be excepted. These are two or three in number. 

 The 1st and 2nd are generally represented by vestiges, only the tubercukim being j)resent : 

 the third is generally long and styliform. From this we may gather that from two to 

 three of the thoracic vertebrae have been transferred to the cervical series by the shifting 

 backward of the sternvim, the sternal ribs disappearing after their severance from the 

 sternum. 



The thoracic ribs are long and slender, and increase in length from before backwards : 

 as a result the thoracic cavity is very spacious. 



The number of thoracic ribs may reach a total of seven pairs, of which six pairs may 

 articulate with the sternum, e. g. Ninox. But this is rare, the number in the majority 



