MORPHOLOaY OF OPISTHOCOMUS CEISTATUS. 61 



there is a tendency to produce three times as many vertebrae as are needed. There 

 are two constrictions in each centrum in all the presacral vertebrae, besides that which 

 afterwards is formed at each intervertebral space. Originally the constriction was, as 

 in fishes, in the middle of each rudimentary centrum (see my paper on the Vertebral 

 Chain in Birds, Proc. Roy. Soc. March 8, 1888, pp. 465-482). This bird agrees with 

 the Fowls and the majority of the Carinatae in having no rudimentary rib on the atlas 

 and axis (PI. VII., at., ax.) : the rest of the cervicals down to the last two have a 

 rudiment, right and left : the distinctness of these riblets as cartilage is very temporary ; 

 they complete the canal for the vertebral artery so far as it extends. These riblets have 

 but a small styloid process in this bird, and in the hinder part of the neck this is lost ; 

 but the ribs break out suddenly in the seventeenth vertebra (PI. VII., c.r."). Those 

 hinder cervicals begin to have an upper spine, and these spines are like those of the 

 dorsals from the fifteenth to the eighteenth. There is scarcely any development of the 

 infero-lateral edge of the cervicals in this bird, tending to protect the carotid artery 

 below. 



The centra of all this series, and of all the dorsals also, are very broad, and all these 

 vertebrae are cylindroidal. The interarticular ligament is normal, being perforated in 

 the middle, the notochord passing through as the "suspensory ligament." Ossification 

 is advancing fast in the 3rd stage, but the centres are not all present ; when complete 

 there is a pair for the neural arch, one for the centrum, and a pair for the riblets. In 

 the axis there are two axial centres, for the centrum of the atlas is fused with it ; the 

 so-called centrum of the atlas is an intercentrum belonging to the junction of the 

 first vertebra to the occipital condyle. Behind this another intercentrum appears 

 belonging to the junction of the atlas with the axis, so that the atlas has three azygous 

 osseous centres. The dorsal vertebrae, with their large quadrate upper spines, show 

 scarcely any outgrowth below, but are unusually broad at that part; their transverse 

 processes are large ; the elevated cup for the capitulum of the rib is normal. Seen 

 from below (PI. IX. fig. 4) the sacral series is spindle-shaped ; in the middle the 

 centra are developed to a great width, for these contain the dilated, hollow sacral part 

 of the myelon. There is a synovial cavity, with an interarticular ligament, between the 

 first of the series and the last dorsal ; for the rest, the cartilaginous centra are fused 

 together, fibrous tissue only appearing between these parts and the hinder half of the 

 uro-sacral region. The transverse lines marking the junction of the centra are curved 

 backwards ; the notochord {nc.) is obscurely moniliform, and there is only one 

 constriction in each centrum. In Stage 1 (PL IX. fig. 5) the spines are confluent, and 

 die out on the last lumbo-sacral, to reappear in the middle of the uro-sacral series. 



The ribs, developed on the first and second, the dorso-sacrals, appear as small 

 remnants on the first lumbo-sacral and the first and second uro-sacrals. The diapo- 

 physes are present throughout the series ; they form strong buttresses to the pre-ilia, 

 become very small in the last sacral proper, and then gradually approximate to the 



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