7. SIBBALDIUS. 



181 



The neural arch is high and massive, and the spine well developed. 

 The lateral processes are large wing-like plates, directed somewhat 

 backwards, with a regularly oval perforation rather above the middle 

 of their base. The dimensions are given in the figure, which is 

 drawn to scale, regardless of perspective. 



Fig. 41. 



Atlas ; anterior surface. 



" The third, fourth, and fifth vertebra) much resemble each other ; 

 they have rounded oblong bodies, high triangular neural canals, 

 spines gradually increasing in length, and well-developed upper and 

 lower transverse processes completelj- united together at the ends, 

 leaving large oval spaces between them. In the sixth the transverse 

 processes do not meet by the space of 3 inches ; and I doubt if they 

 ever would meet in the process of growth, on account of the different 

 planes of their ends. The upper one is long, with its flat surface 

 almost vertical ; the lower one, short and broad, with a stout conical 

 tuberosity projecting forwards and downwards from its base, turns 

 so completely on itself that its broad terminal end is directed hori- 

 zontally ; it is, moreover, very nearly complete. The peculiar form 

 of this process is highly characteristic of all the specimens I have 

 examined of the genus SilihahUus, though it is best marked in the 

 one under consideration, being the most mature. It should be men- 

 tioned that, when the series is placed together, a gradual apjiroach 

 to its form is seen in the lower processes of the antecedent vertebrae. 

 The seventh cerN^cal vertebra lias no trace of an inferior transverse 

 process. The thicknesses of the under surface of the bodies of the 

 liist five cervical vertebra), and of the first two dorsal (without the 

 epiphyses), are respectively 1-5, 1-3, 1-4, 1-5, 2, '2-2o, and 2-5 inches. 

 The width of the first dorsal vertebra across the transverse processes 

 is exactly the same as that of the last cer^•ical, 23" ; the second is 3" 

 less. The transverse processes of the posterior dorsal and of the 

 lumbar vertebra; arc very broad in the antero-posterior direction. 



