24 ME. W. P. PTCEAFT ON THE 



The seventh pair of thoracic ribs are frequently found only in minute traces of their 

 sternal segments (figs. B, C, s.r. 7, p. 28), at times even these are wanting. Sometimes 

 traces may be found of vertebral as well as sternal segments, and still more rarely the 

 whole rib may be found, though it fails to reach the sternum (fig. A, s.r. 7, p. 28). In 

 three specimens in the Museum Collection the seventh rib is well preserved — Bubo 

 magellcmicus (S.r. 7, fig. A, p. 28), a Nyctala 98.22, and a Ninox 1332 a. 



The constancy in the number of the cervical and thoracic vertebrae and the fixed point 

 afforded by the sacral vertebrae enable one to determine, with some certainty, the 

 number of the lumbar and lumbo-sacral series. 



Only five skeletons in the National Collection have as many as four lumbar vertebrae. 

 At first sight, two skeletons of this five ajjpear to have five lumbars, indistinguishable 

 one from another ; as a matter of fact, the first vertebra belongs to the thoracic series, as 

 is proved by the vestiges of the 7th pair of sternal ribs. Thus, it is absolutely necessary 

 to fix the last thoracic before attempting to count the lumbar vertebrte ; and this 

 fixation is often only possible through the presence of the sternal segments of the 

 thoracic ribs just referred to. 



The maximum number then of lumbar vertebrae is four, and this is now rarely attained ; 

 in all, save five, of the skeletons under my charge, only three are present. Since there is no 

 suspicion whatever of the fusion or reduction by crowding of two contiguous vertebrae in 

 this series, I gather that the loss of the vertebrae in the series is due to excalation. It is 

 possible, in some instances, that the 4th lumbar has, by loss of its par- and di-apophyseal 

 processes, become merged in the lumbo-sacral series ; but in this case, it is still a fact 

 that a vertebra has been lost, only it is a lumbo-sacral and not a lumbar that is missing. 



The lumbo-sacral vertebrae are three in number, but in five cases they are reduced to 

 two, as in fig. C, 1-2, p. 28. Instances of this can be seen in the annexed table. 

 Excalation appears to have been at work here again, since there is no trace of the 

 reduction and fusion of contiguous centra. 



The primitive sacrals are two in number {s.v., figs. A, B, p. 28), but the ribs, either of 

 the 1st or 2nd pair, are occasionally wanting. 



The sacro-caudal are three or four in number (1-3, fig. C, p. 28). Occasionally the 

 vertebra corresponding to the first free caudal fuses with the last sacro-caudal, so that 

 the synsacrum actually projects beyond the limits of the mesial borders of the post- 

 acetabular ilia. 



The post-synsacral vertebrae or free caudal vertebrae are either 7 or 8 in number. 

 In very young birds, e. g. Speotyto, there are 9 free caudals, the additional vertebra 

 lying in front of the pygostyle. It is small and wedge-shaped, and gradually dimin- 

 ishing, partly by the mutual pressure of vertebrae on either side. Later in development 

 all trace of this vertebra disappears, owing to its fusion with the pygostyle. 



Proof of a reduction in the number of presacral vertebrae may be equally well demon- 

 strated if the component numbers of the various series be disregarded and the total 

 number of vertebrae be counted between two fixed points — the atlas and second sacral 

 vertebrae. As is shown in the following table, the number counted in this manner varies 

 between 28 and 30, whilst the variation in the total number ranges between 39 and 42. 



