106 BULLETIN UNITED STATES GEOLOGICAL SURVEY. [Vol.YI. 



and tlie first upon tlie last sacral vertebra. The articular facets upon 

 the centra vary in shape throughout the series ; that upon the first is 

 long transversely, with a double convexity so arranged as to accommo- 

 date itself to the one on the extremity of the sacrum ; they soon become 

 uniform, to pass to the subcircular one existing between the last verte- 

 bra and the pygostyle, on which it is concave. 



The pleurapophyses and parapophyses are very rudimentary or en- 

 tirely suppressed. Each vertebra bears a prominent neural spine, which, 

 from the first to the sixth, inclusive, is bifurcated j in the last two it ajD- 

 pears as a mere primitive knobule. The transverse processes are all 

 deflected downwards and outwards, very small in the first and still more 

 so in the last ; are largest in the fifth and sixth. Prezygapophyses are 

 well marked ; they reach forwards and articulate with the feebly devel- 

 oped postzygapophyses. In a few of the posterior segments there ap- 

 pears to be an effort on the part of the neurapophyses to overlap the 

 vertebra next beyond them. The neural canal is pervious throughout, 

 commencing in the first with a calibre equal to that in the end of the 

 sacrum ; it gradually diminishes, and terminates in a minute, blind, con- 

 ical socket in the pygostyle. Hypapophyses are produced downwards 

 in a few of the ultimate vertebrae. They hook forwards and articulate 

 with the centrum of the vertebra next beyond them. Sometimes they 

 are observed to be free, or rather resting upon a facette on the anterior 

 margin of one centrum and extending over to the anterior margin of the 

 centrum of the vertebra anterior to it, to meet a similar facette, as a 

 tiny styliform process. The spinal column is completed posteriorly by 

 the pygostyle — that j)loughshare-shaped segment that articulates with 

 the last coccygeal vertebra. Above its cup-shaped facet this bone arises 

 as a laterally compressed plate, extending backwards and bifurcated at 

 its extremity, as if to imitate the neural spines of the vertebrae of the 

 series of which it is an ultimate api)endage. Below the facet it projects 

 forwards and completes the median sequence of hypai)ophyses of the 

 centra, being rather larger than any of them. The posterior curve is 

 simply inflected downwards and forwards from its apex. 



The scapular arch — (See PI. I).* — The three elements that constitute 

 this arch are all represented and independent bones; the coracoids ar- 

 ticulate with the sternum and scapulae ; coracoids and clavicle, con- 

 nected by ligaments, lend their share to form or strengthen the shoulder- 

 joints. The coracoid, comparatively large and strong, forms in the 

 usual manner an arthrodial joint of restricted movement with the ster- 

 num, its lower end being in the coracoid groove on the anterior part of 

 that bone. The inner angle of its base is about 2 millimetres from the 

 mesial line, and 4 millimetres interveniug between it and its fellow of 



"^It will be seen that in this figure, correspondiDg limbs, and other parts that are 

 alike on either side of the body, have not been reproduced, it being thought the bet- 

 ter way, as the bones on the side towards the observer would necessarily obscure the 

 more remote one, complicate the figure, and show nothing additional. 



