1888.] 



On the Vertebral Chain of Birds. 



479 



processes (diapophyses and parapophyses) ; the arches by arrested ribs — ■ 

 " pleuroids." As a rule, in the CarinataD, these are not developed on 

 the axis and atlas ; but in the Anatidae, as in the Ratitae, generally, 

 they are found in them also. The arch on the atlas is a strong but 

 narrow bar ; in the Cygnet of a month old there is in it a styloid 

 bony rib, placed subvertically. The rest are larger, are horizontally 

 placed, and have a free styloid end, which in many cases almost 

 reaches to the end of the centrum of the next vertebra. These 

 riblets have but little primary independence as cartilages ; but they 

 ossify separately; they are clavate, and this clubbed fore-end has 

 thus no distinction of " capitulum " and " tuberculum," although 

 the lower edge answers to the one, and the upper to the other. 



In the twenty-second vertebra the styloid part is lost, and only a 

 broad vertical bridge is developed by the "pleuroid;" in the twenty- 

 third only a narrow bridge, like that on the atlas, but stouter. On the 

 twenty-fourth and twenty-fifth the ribs are segmented off, have double 

 heads, and remain free, although they do not form a perfect arch by 

 reaching the sternum ; indeed the last but one is very short. In these 

 two vertebrae the facet for the capitulum is on the centrum, opposite 

 the lower part of the facet of the centrum ; that for the tuberculum 

 is on the diapophysis. Thence along the five free dorsals and the 

 two first dorso-sacrals, the joint for the tuberculum (the parapophysis) 

 gets gradually higher, so that in the two last it lies over where the 

 suture was between the centrum and neurapophysis. The developed 

 ribs of the third and fourth sacral (dorso- sacral), have lost their 

 capitulum, and articulate only by their tuberculum on the diapo- 

 physis. 



The last three vertebrae of the seven that buttress the pre-ilia, have 

 only a generalised mass, right and left ; and on the next four, the 

 true sacrals, these are either gone, or reduced, to mere prickles. The 

 twelfth and thirteenth have strong pleural bars, not segmented off in 

 the cartilaginous condition, but they are ossified as distinct bars ; 

 these coalesce with the centrum and diapophysis. Behind these, in the 

 Cygnet, there are no " pleuroids," but in a recently hatched Duckling 

 (Anas boschas, domesticus), I find jive pairs of these little rib-bars to 

 the fore-half of the Urosacral series. 



Thus there are thirty-four pairs of ribs, rudimentary or developed, 

 without a break, in the Common Swan, and then an attempt at 

 forming a new series behind the sacral nerves. Also, let it be noticed, 

 that the first two pairs of pleuroids, or rib-rudiments, arise from inter- 

 centra, whilst the last two of the twenty-nine have lost their capitu- 

 lum, or primary head, and are articulated by their tuberculum or 

 secondary head to the diapophysis, an outgrowth of the neural arch 

 (neuroid). 



Thus we have in a single vertebral chain an epitome of the history 



