330 MR. W. K. PARKER ON THE OSTEOLOGY OF BAL^NICEPS REX. 



If we compare together the haemal canals arising from vertebral centrums, without 

 the help and intervention of heemapophyses and haemal spines, we must come to the 

 conclusion that they are essentially formed in one manner. The posterior abdominal 

 and caudal arches of the Sturgeon, Herring, Dory, Sea-bream [PageUus centrodon- 

 tus), the open caudal passages in the Ophidia, the closed arches in the Slow- 

 worm, and lastly, the carotid canals in the neck of birds, thus have a homological 

 unity. 



Nor does the structure of the anterior vertebrae of the siluroid fishes {Silurus, Bagrus) 

 present anything essentially new ; for, as Professor Owen himself has shown, the pass- 

 age for the aorta along the base of the coalesced centrums of these fish, is formed " by 

 exogenous ossification in and from the lower part of the outer layer of the capsule of 

 the notochord." 



In distinguishing between diapophyses and parapophyses in the vertebrae of birds, we 

 have to bear in mind the embryological fact, that during ossification " the bases of the 

 separately ossifying neurapophyses extend over much of the centrum, and soon coalesce 

 with it." But this fact is quite in harmony with what we find in the adult bird, for the 

 bases of the upper transverse processes or ' diapophyses,' exogenous growths in the 

 corporal vertebra from the neural arch, expand over the upper half of the sides of the 

 centrum. The distinct ossification of the cortical part of the centrum in the atlas of 

 birds is a pretty good guide as to what processes may be called ' parapophyses,' viz. 

 just above the lower margin of the centrum, as well as at and beneath that margin. 



The ' centrum ' in oviparous vertebrata is almost as rich in processes having a teleo- 

 logical meaning as the ' neurapophyses;' yet parts arising from the neural arch are as 

 a rule more definite in shape and function, and are consequently more easily classified 

 than those spurs and processes which arise from the body of a vertebra. 



So that the single median processes called ' hypapophyses ' may be seen by tracing 

 them in successive vertebrae in birds, to be formed by the gradual coalescence of par- 

 apophyses that have become in each succeeding bone nearer and nearer each other. Or 

 the opposite method may be used in tracing them, and then we shall see a hypapophysis 

 gradually bifurcating, the bifurcations becoming sessile, and forming distinct and true 

 parapophyses. The Penguin is one of the best birds in which to see the ' specific unity ' 

 of these exogenous processes of the lower part of the centrum. In this bird {Eudyptes 

 demersa) the two last cervicals have distinct floating ribs, and these vertebrae are fol- 

 lowed by seven distinct dorsals, having the ' opisthocoelian ' mode of articulation of 

 their centrums. 



In the penultimate cervical, which is very broad beneath, there is a pair of anterior 

 cupped parapophyses for the heads of the small ribs, and a small mesial process between 

 and a little behind these. In the last cervical, in addition to these three processes, 

 there is a pair of broad thin widely divaricating processes passing backwards and out- 

 wards from the mesial one. In the first dorsal the anterior articulating processes are 

 higher up on the side of the centrum, and the mesial process has vanished from between 



